Okay, I'll put it this way, then:
The player knows, thanks to the manual, why Manannan hates humans and how he fears what he once saw as his ultimate end. Do we expect that Gwydion knows this because we do? No, of course not.
Likewise, in the original game, you never get confirmation in-game of Manannan's plans (unless he DOES kill you, which, obviously, didn't happen since Alex lives on). So really we've already added to what Alex knows with his confirmation of knowing Manannan killed previous slaves. There are messages that give you relay Alex/Gwydion's suspicions that his master has some vile plans for him, but unless there's a message I've completely forgotten or never found, you, as the player/as Gwydion are never actually TOLD that he knows specifically that Manannan has killed before and plans to kill him. Most if not all of that information is in the manual, not the game.
So, that's what I mean by player knowledge does not equal character knowledge.