Well, even for Yonkou fana it's far fetched to use this data to compare Aokiji with regular YC1. We don't know Aokiji's truth intentions, although with the way the flashback is presented, it doesn't look like he had been seeking to join Blackbeard with ulterior motives but rather a casual circumstance.
We have rocks pirates as a precedent that there can theoretically very well be Yonkou tier working for or under other Yonkou.
Ultimately if I am honest, although it's definitely not a good look, it doesn't really change Aokiji's power level because we know he fought Akainu and we have seen what Ryokugu does to YC's. And he's leaving anyway, his talk with Garp sounds like he's gonna rediscover himself once again.