Whatever it is, it's just not canonically possible for Renji to become stronger than Yamamoto in any way. It's extremely consistently portrayed that Yamamoto is a static pillar of strength in the story, far beyond just CFYOW, and Kubo has explicitly confirmed that veteran captains are static in power several times (look below). Furthermore, it is a significant narrative point in CFYOW that things work in this grounded way, where (most) CFYOW captains don't powercliff Yamamoto. It wouldn't be a simple statement getting retconned, these narratives which are consistently established through the manga, guidebooks, interviews, and novels would be completely broken. Also, there is no indication Kubo has radically changed his mind on the story and how things work in it, based on his Klub Outside answers.
Shunsui is still relevant, and he'll go on to fight a Post-Auschwahlen Royal Guard member. At the start of the TYBW, Kubo explicitly said this about him:
Q (some Shonen Jump guy): The senior captains won't undergo changes, so Kyoraku hasn't changed either?
Kubo: That's correct. Captains who have been in their positions for a long time don't tend to change much. They've gone through many battles in the past. However, in the future battles, you will see different and new sides. I believe that Kyoraku will use Bankai. Also, there is a certain significance to the haori that Shunsui wears.
The most consistent and logical way of looking at this is that we made a mistake analyzing the characters or scene, and Renji didn't become that strong. Even contextually, based on the stuff we saw in the last cour, Rukia is narratively shown to be relative to Renji, and she wasn't completely wrecking As Nodt, a regular Sternritter in every way. She didn't flick him away, which she'd clearly be able to do if she was even somewhat relative to Renji.
I saw people discussing this on twitter, and based on his explicit statements from the interview he did with Viz, it seems unlikely that the tweet is fully accurate.