My understanding was that he did
not know what happened, and didn’t really care anyways.
The Acathla plan was not something that Spike was really on board with, as it had caused Dru and Angelus to become closer. I believe he also has reservations about destroying the world, since he seems to just revel in having “fun” and doing whatever he wants. He may say something directly about this, but it’s been a minute since I’ve watched the entire second season so I can’t say for certain.
Nobody outside of the Scooby gang learns what really happened that night, and the Scoobies themselves don’t learn until quite a few episodes after the fact. Angel himself doesn’t really know what happened or why he was resurrected at this point in time, so it’s unlikely that he’s going to tell anyone about it either. Any routes of information that would have delivered this to Spike were out of the equation.
As for why he isn’t surprised? Angelus was ensouled before, and during Spike’s after-lifetime. He’s seen the whole shebang happen already, so it stands to reason that he wouldn’t be all that shocked that Angel was back again. Since he didn’t stick around to see the deal with Acathla, for all he knows Angel was re-ensouled and stopped the process himself.
But that’s just my take on it!
