Iirc in the original book, the planet of the apes wasn’t actually earth, but when the guy goes back to earth at the end it’s been taken over by apes. Or something like that.

Actually this is 100% explained in the prequel, Spaceballs.

Well… You see, it all started with a taxifing clown, a con with a big fucking rocket, and France…

When they blew up the statue the top bit flew through space and landed on the planet. The guy literally tells you right after seeing it.