We don't really know for certain what the etymology of Prussia is.

One etymology, that it arises from the phrase po Rus' '[those living] by the Rus', isn't well accepted, and I don't think there's much to it. The correspondences don't work out very well at all, and the fact is that Rus' isn't of Slavic origin in the first place, suggesting a more recent date for the etymology than is probably justifiable.

Importantly, we see ū in Baltic words for Prussia (Prūsai and Prūsija in Lithuanian and Latvian respectively), and, assuming these names are, in the strict sense, cognate, we should see Slavic (or at least Polish and Russian) reflexes with y. But we don't see that: the native Russian form has the vowel u. I should also point out that Lithuanian has Rusija, with a short u, not a long one like in their word for Prussia.

The issue of the o in po is also quite problematic. We do have words that are unambiguously derived from this sort of construction, like Pomerania < *po morje 'by the sea', and they retain the o. Thus, we should expect Prussia to retain the o too, if it comes from the same kind of construction.

What's more, the po Rusi etymology relies on a German form Borussen, which Max Vasmer links to a Greek spelling αοροῦσκοι, putting the form at quite some remove from any native terms.

I think the weight of the current evidence is that we're really pretty much uncertain about the source of Prussia, and it can't be reliably demonstrated to have any relationship with Russia, which has a well-established etymology. The po Rusi etymology (which some I've seen pushed on Reddit somewhat commonly) is problematic on a number of counts, and should be rejected.

It's pretty certain that Russia derives comes by way of a North Germanic term for rowers.