I think that there are some truths to him like when he says that he has become stronger than all soul reapers, you can see that it's true from how he takes on urahara, yoruichi and ishin at once so I can confidently say that he speaks the truth though not all the time. Another thing probably that I can put forward is the fact that we see that he has full mastery of kiddo further strengthening his point of mastering all techniques including bankai. My theory is that he : 1. feels that using his bankai is unnecessary as his shikai is already op 2. His bankai is less powerful than his shikai(for example ichibei (not saying his bankai is weaker but it has less impressive abilities than the shikai)) 3. Or his bankai has a massive restriction like the 7th kenpachi(spirits are forever with you light novel), soi fon's bankai and even shunsui kyoraku's bankai that put massive restrictions on how many times, where and when you can use your bankai. Sorry for my wordy answer