The situation is different. Having immunity readily available on a leader limited the design space too much. For this same reason, Emhyr was nerfed. Regardless, let's delve into this particular comparison.
With an immune Slyzard, you could just keep eating units. Saesenthessis can only receive a one time boost. The difference between the two decks is that Slyzard is using units, while Saesenthessis is using spells and artifacts. This distinction is important because you don't need Saesenthessis to have a spell/artifact/control deck. As such, changing her changes little for those decks. There is only one ST deck that goes all in with Saesenthessis and that's only competitive against control decks. CDPR shouldn't make a habit out of nerfing cards because of fringe cases. Not only does it limit creativity but it also makes the meta more stale.
Immune, as a concept, could be redesigned, though. But that wouldn't be limited to Saesenthessis. Incidentally, Monsters can do the same trick with Werewolves.
Irrelevant. You were asking for an Eredin nerf because Slyzard could not be interacted with (besides Scorch). If you want to handbuff Saesy, you need to put units on the board and those units can be interacted with. Because of this, all-in decks don't use handbuff, which renders your argument moot.