Not necessarily.

A. They might have really liked the flavor and/or mechanics on the UA version and the homebrew version was different enough that it wasn't as interesting to them

B. OP might have taken long enough researching Mythic and it's alternatives that the player had time to reconsider their choice. The player might have been split between the two anyway

C. The player might have realized that running a homebrew/UA class was going to be a lot of extra work for themselves and/or the GM, so they decided to do something easier

Honestly it's fine as long as everyone is in a similar power band. All you do is scale up encounter difficulty slightly by adding monster abilities or extra opponents. All power levels are relative to the campaign's conditions, after all.

What sucks is when one person is vastly stronger than their peers and sees no problem with this. Those people never learned to share and should be shunned.