Well, true, but presumably if she had friends skilled enough to install one in Chervignon in the first place, they wouldn't have suddenly lost that knowledge if Alaric were to call upon them to install one in Coroth as well. The real question would be, would Alaric be willing to have them do so, considering anyone with the skills necessary to create a TP in Coroth would also, after the fact, be completely familiar with its coordinates and presumably able to pop in and out at will? (Unless, of course, he sets up further protections around it.) He'd probably want to delegate the task to someone who both has the skill and who he trusts completely, which may or may not be the same circle of friends Constanza called upon when she built the one in Chervignon.
I don't think the knowledge is as specialized as, say, healing though. I.E., there's no evidence from the books that one has to be genetically "hardwired" to be able to express that trait, so any Deryni who has enough formal training would probably be taught the skill at some point. Otherwise it would have died out long since, and it's doubtful there'd have ever been many residential Transfer Portals such as the ones in Tre-Arilan or the MacRories' residences, yet Deryni Magic
mentions private residences of Deryni as being logical places to find Transfer Portals, whether active or inactive due to having been forgotten and/or unused. There could well be Transfer Portals in other Deryni homes throughout Gwynedd, including in Corwyn itself if it's served as a refuge for Deryni families over the years. If so, it's doubtful they'd go around publicizing the fact. If you were a member of a persecuted and technically illegal population, would you officially register your secret Transfer Portal, even to a Duke who is one of your own kind? What he doesn't know about can't get you or him into trouble with the Deryni-hunting Church of Gwynedd.
Plus, they've probably been set ablaze or otherwise ruined or disabled through disuse throughout the years of Deryni persecution, as families with the trait were discovered, burned at the stake, and their properties either burned, torn down, or given to human families. Plus, a TP is useless unless you know another location to TP to, so even a Deryni family in hiding might have their TP go inactive because they don't have a clue who the other Deryni families are, much less what their TP coordinates are. Sir Stefan could well have such a TP somewhere on the grounds of Kestrel Mote, and even be vaguely awere of where it is ("Oh yeah, that strange cobblestone in the barn that tingles!"), but unless he was ever shown how to use it or where it could take him, he might as well not have one at all. Would a man as paranoid of being found out as Sir Ranulf was have taught his family how to use a Transfer Portal if he knew of one at his manor? Doubtful. Or at best, he'd have shown them only so they could use it escape Kestrel Mote at dire need, showing them whatever other secret TP location his own father may have passed down to him for such a contingency, but forbidden them to use it otherwise.
So would it take skill and training to build one? Sure, and granted, that's a bit hard to find west of Coroth due to Deryni having to live underground for two centuries. Is it a nearly lost art, or one so rare only a handful of Deryni could do it? I really don't see evidence for that in the books. We only know of a handful of specific locations, but that's because those are the only ones that were relevant to the story KK was telling, not because those are the only locations that exist, or because of a limited number of Deryni capable of building them. Even someone like Sofiana probably knows only a handful of TP locations at best, since people would tend to keep such information secret from all but their closest friends. (Would you
want just anyone being able to pop into your house at any time? Heck, as much as I love my family, I still wouldn't want even my own mom to be able to just pop into my house without notice except in case of dire emergency! What if the house was a wreck? If I was in the shower? If the DH and I were having an intimate moment mere meters away?
From Arilan's working in HD, it appears the main factors involved in building one would be the knowledge of how to create the Portal plus having enough assistance to draw upon (preferably Deryni) for the massive amounts of energy needed for the working. Oh, and it needs to be set in rock or earth of some sort, not just on any surface. A plank floor wouldn't do. But Arilan built one in a battlefield pavilion on bare ground, with assistance from people who didn't have previous knowledge of how to build one themselves. He wasn't born with that knowledge; someone would've trained him. And presumably whoever trained him also trained others. I imagine it would be considered fairly standard, if a bit advanced, training for a formally trained Deryni (i.e, not something a Deryni child or even possibly an adolescent would know, but definitely something Alyce would've gotten around to teaching her son eventually, or found someone else to teach him, if she'd survived to his adulthood), but there's certainly no dearth of trained Deryni in the lands east of Gwynedd where they've not been torched on a regular basis for two centuries.