It's guaranteed to be depth-first traversal. (But the file order in each individual directory is of course unspecified.)
The first hint is the existence of .disable_recursion_pending(). It would be hard to implement for a non-DFS approach (you'd have to store an array of bools for the current directory?).
Then there's [fs.rec.dir.itr.members]/21 that directly requires what is essentially DFS.