>>21351350And? Mirror doesn't mean opposite land.
While I venture that there is no ironclad PROOF that Ash has won all the gyms so far that his prime counterpart did, it's safe to assume. He's even traveling with the exact same people. Mirror Serena is a terrific battler, is all. Or Ash is less competent, but at least has still accomplished most of what prime Ash has.
Who knows? Maybe Mirror Ash even did things prime Ash hasn't.
Like traveling with Hilda and Rosa