It was from a Kubo interview earlier in the year before the final arc started...
If you want to read that part of Kubo's interview, click the next Spoiler link:
Q: Has Renji trained hard to become stronger?
Q: Have Other people trained as well?
Q: So they wouldn’t be troubled by even Aizen-level opponents?
Kubo: They’re not quite at that level (laughs). What can I say, they’ve become stronger. But even so, they haven’t gotten to the point where they’re not worried about Aizen-level opponents (laughs).
Q: Will they display new abilities or techniques in the new arc?
Kubo: That will be revealed. Renji’s bankai has already changed.