Seeing the Tories confidently declare they will negotiate a series of treaties that will be in the UK's favour, only to be shoved around by anyone and everyone and generally being made to look like incompetent oafs is a pretty textbook example of schadenfreude no?
Pedantry: what you describe is just "comeuppance". To be schadenfreude, you have to be taking joy in it.
You can have schadenfreude without comeuppance, though that can verge into just plain sadism or gloating. Schadenfreude usually has a connotation that the misfortune is deserved.
So yes, Remain voters (and those who sympathize with them) may definitely be experiencing some schadenfreude watching the Tories hoist by their own petard.
Yeah it wasn’t explicitly stated but I don’t think it’s a stretch to assume the person who used the term in this thread is enjoying it slightly, given the statement that followed it
They were the party most loudly in support of Brexit, voting for it in the largest numbers, and they were also the ones who made it happen. If that doesn't make them "natural Brexiters" then the phrase "natural Brexiters" is meaningless. They are the Brexiters, more than anyone else.