These two statements contradict each other

[Movie Bowser]

Big Buzzy Beetle

Mario must defeat it to continue his adventure.

It is also the only optional boss in the game.

If I "must" defeat it, then how is it "optional"?

 

[Waluigi Time]

Based on the layout of World 1, it looks like you can beat the game while skipping Hither Thither Hill entirely.

 

[Koopa con Carne]

you can circumvent the boss battle, but once you're in, you have no other option but to go through with it. seems like that's what the phrasing is going for. could use a rewrite though; there's no boss battle in this game you can escape from, so i don't see why this notion has to be noted here in particular.

note that i don't remember specifics from my playthrough. just attempting to elucidate the above.