Honestly, there's really no way to justify the teleporting away other than with "well, actually, they technically could to it if they had the right abilities setup". Having said so, that's really more of a setup than any other game with this annoying trope in it does - in most other games, you can't point to the gameplay and say "see? The abilities, right there?", you just have to take it.
I mean, honestly, I don't think the presence of in-game Teleport actually matters to the discussion at all? If we want to be technical, we should start with how none of those enemies actually had Teleport in their builds to begin with, so even going by in-game logic they shouldn't have been able to use it.
In any case, the game doesn't treat the casual ability to teleport around as a part of the setting. We don't have a scene of Ramza teleporting behind Wiegraf and unsheathing his katana, we don't have Agrath threatening the captive Corpse Brigade soldier that escape is useless, they have people positioned to watch over all possible teleport locations. And so on.
It is possible to imagine a game where it fits. One about ninjas, perhaps, waging a shadow war mostly fought in quick skirmishes where one side gains an advantage and another retreats because everyone involved is a master of escape. FFT is not such a game. It uses teleport as a shortcut to "they escaped", and I don't think the ability to get teleport reflects on it one way or another.
This whole discussion about how artificial and frustrating the repeated escapes despite you clearly defeating them in gameplay feel make me think about how Fabula Ultima, a JRPG-inspired TTRPG, handles this trope. Basically, Villains (like Wiegraf and Isilud, important NPCs with goals that oppose ours) have a certain number of Ultima Points, which they can spend to reroll checks and escape from the scene after being reduced to 0 HP.
Double Cross has a similar take on it: there is a special enemy-only power that allows escape from battle once per adventure (so it is strictly limited, you can't repeatedly escape the heroes). The thing about it is that it's fully integrated into the combat system, with proper stats tying into standard rules. So, that power has Timing: Auto. Normally, it's a bad thing because it means it can be used at any time, up to and including after the damage roll was made but before damage was applied, allowing the enemy to exit stage left at the last possible moment... Except there are other powers, available to players, which inflict a status effect that prevents the target from using Auto powers. So, if an enemy tries to pull this shit on you, you can, in fact, say, "No. No you don't".
(Now, there are various ways to enable the enemy to use the escape power anyway as there are standard powers to remove status effects and an enemy power that grants outright immunity to a given status effect, but it's still funny that cutscene escape can just be blocked if the GM forgot to build a proper combo for that.)
