Those are enemies/monsters
from the future. Why wouldn't they be there? I have no idea what connection you're making. They aren't fought anywhere else except the future. So no, Squall and the others have not seen them before except in that castle.
Think for a second. Those GFs are only drawable from Ultimecia's minions if you've failed to obtain them from their proper locations earlier in the game. In essence, in a branch where Squall and the others literally fail to use or obtain any GFs throughout the whole story, they find themselves there to be obtained. Ultimecia doesn't "have" every GF. Because they're naturally meant to be elsewhere in the game. That's only the game's failsafe for people who may have skipped them on their first playthrough.
The game
literally says so. I just quoted it's profile to you and what Ulti says. Griever didn't exist, it's made up until she materialized it from Squall's mind.
That website is literally riddled with misinformation. Again, Witches are not immortal. They have normal human lifespans. They can't die until they pass on their powers but that doesn't make them immortal. That makes them wander in a state of suffering on the brink of death, looking for the first woman they see to pass on their power before they expire.
The FFVIII Ultimania literally makes that point explicit; witches are not immortal. They have normal human lifespans.