Uryu still did better than Renji at that point. Mayuri came in later with prep time and a counter to Szayel's powers.
A captain is a captain, they need to be able to show that they are able to at least be a certain strength or quality to attain that position. Whether he got a temporary boost or not, he still won. You can call it plot driven, but that would be for all the powers shown later on.
Blut and Sklaverei are natural quincy techniques, he had his schrift before Yhwach, but never unlocked it. He wasn't given these powers, he only had them unlocked.
The only thing he was given were amps from aushwalen which did help him get stronger.
Renji getting a "real" bankai can also be argued to be a plot-driven power since he was always taking L's most of the series.
Renji:
Lost to Byakuya
Lost to Szayellaporo
Lost to As Nodt (small confrontation)
