Was it ever stated that Rox Pirates were considered unstoppable instead of just very strong and threatening beacuse of their terrorist-like profile? I can't recall right now.
Anyways, I think the whole idea of "top tier" is dependant on the average power level of the rest of people, so if the world was overall weaker back then (just like Luffy and Blackbeard, maybe Sakazuki, will likely become more powerful than anything ever seen) then maybe Xebec was the first top tier ever according to current Era's standards. But I personally disagree, I don't share your point of view and I think that Carmel lecturing Linlin as a potential admiral might point to "top tiers" being a thing already back then.