Is it possible a previous space probe seeded the atmosphere though? For example, did the Soviets sterilize all the Venera probes perfectly?
Even if this were the case, it would still be quite amazing.
That would be a amazing sensation. Terraforming would then be much, much easier, than expected. But I doubt it.
I think it was maybe Carl Sagan who said (something like): if we find life in our solar system it's probably game over for us, as it means the great filter is likely ahead of us, not behind us.
Though, you can take one look at the world right now, and not see how easy it would be to just destroy it if we aren't careful (and we're not being careful).
Though it's quite possible that if there is something on Venus or Mars it's Earth-origined, since conditions here 4 billion years ago look better for abiogenesis.
If there are several filters, then they may differ in effectiveness by many multiples, and then it's semantics as to whether you say any beyond the largest are "great".
I think a more interesting concept would be exchange of microbes between Venus and Earth at some point in their distant past, and then could we might be able to date that (those) event (s) via their molecular clocks (https://en.m.wikipedia.org/wiki/Molecular_clock)