I dont consider megalovania to be SANS’ES theme

I consider it to be YOUR boss theme

The humble Song that Plays When You Verses Sans:

Thats not canon cus its in the 10th anniversary live stream

But if it is by canonicity, then we shouldn't be talking about musical 4th-wall motifs and tracks
The only important diegetical music tracks in the UNDERTALE OST are Death by Glamour, Spider Dance and Long Elevator

Define important. I can think of a few diegetical tracks that I believe are more important than Long Elevator.

bro tf u on

Probably sniffed up some Determination again

Megalovania

The 10th anniversary content is in a state of half-canonicity

Megalovania

Megalo Strike Back

Whats a diegetical music track

It's a song that actually plays in the context of the game.
For instance, a song like CORE probably wouldn't be audible to the characters in game, but Death by Glamour is.

There’s no reason to believe this and you’re far from the first to do so

Well if Death by Glamour isn't diagetic, then
why does Mettaton just dance to no music?
Is he stupid?

or are you?

How can you say that¿¿
Long :jacko: Elevator is the most important track in the game
(/j, if you didn't understand)

how is spider dance diagetic exactly?

The flavor text says that the spiders dance along I’m pretty sure

ohok

or are you?.

Cue vsauce theme