Asshai may not be expansionist(as far as we know), but that doesn't mean there might not be people there with as much or more magic than we have. This is the same Asshai where Melisandre claims to hail from(or where she learned her hax), the same Asshai that would have had a head-start on the rest of Planetos(though for all we know there might be similar such cities in Ulthos or Sothoryos or some continent we haven't even heard of, let alone seen).
Also, when was the last time DP even said that? I'm also quite sure he did not say at the literal top, but among the top, because the first would make no sense in the context of someone we know is stronger, and that is Bloodraven. We have no clue who else might be out there that's at his level. The crux of the matter is we have no clue how far Planetos actually stretches, it might just be Sothoryos and Ulthos that stretch on and on, or there might be some sort of Americas type deal, we simply do not know.
I think it beyond arrogant of us to assume we're at the top of the shitheap and rest on our laurels, seeing as people like Bloodraven were out there long before Viserys was even a twinkle in good ol' Mad King Aerys' eyes. From the locales we
know, there's Leng and its god-empresses who consort with
Old Ones, Yi Ti with its ancient history and lore(and this absolutely batshit
monster), Valyria and whatever remnants survived, like what few liches might have persisted past the Doom. I feel it's pertinent I mention Asshai again, cause seriously, Asshai.
We might be near near and around the top of Planetos, but to assume we are the
best is pure hubris.