That is a meme way to describe it but yea that's it, but also not.
I usually go with the brain dead answer of post literally means after so post-x is whatever happened after the genre has been defined and whatever is post-x is just trying to break said genre out of that mold
When it comes to "post" it wants to create an atmosphere more than anything. Like, this isn't a song that you can really appreciate on the car ride to work, it's a song that you have to sit down, put the headphones on, get comfy, and just get lost in the soundscape that the artist is attempting to create. Daughters is Post-rock but it is way different than Swan's latest releases
youtube.com/watch?v=tSl4w6PXVbY
Moth Gatherer is a solid Post-Metal band and it doesn't really suit that idea of "prog with x instead of y"
youtube.com/watch?v=KTxfkedv7pc
My point being, post-x just wants you to get lost in a soundscape more than anything