Shinryu, think back to when Grimmjow fought Shinji. When Shinji launched his Cero at Grimmjow, Grimmjow was able to minimize the damage done by launching a mini-counter Cero. The situation with Bazz-B's flames is similar. Bazz-B's flames, in conjunction with his, NaNaNa's, and Äs Nödt's Blut Vene were a defense barely strong enough to keep them from getting roasted by Yamamoto's flames. If Bazz-B's flames were on par with old man genocide's, then Yama would've been hurt too.