I don't know if Apollo made an appearance in TSATS, but if he did, he was probably butchered too, because Apollo WOULD NOT let his children go to Tartarus.
If he heard what Will and Nico were up to, he'd kidnap them and put them safely in his sun palace. He'd be like, 'OH MY GOD. NO YOU'RE NOT GOING. LEAVE THIS TO ME.'
He'd try to talk to Hades about rescuing Bob, but if he learnt that Hades was the one who sent the message?
Boy oh boy....................RUN FOR COVER.
Los Angeles wouldn't just have an earthquake, it would be decimated entirely. Apollo would not hold back, not even if Hades was one of the Big Three. Multiple gods would have to intervene, probably.
'WHAT DO YOU MEAN YOU SENT YOUR OWN CHILD TO TARTARUS YOU ABSOLUTE (REDACTED REDACTED REDACTED). AND MY SON WAS GOING TOO? YOU LITTLE (REDACTED)-'
Apollo would not let Will or Nico go Tartarus. No way in hell.