Well see and that's what kinda makes this a bit frustrating, and in my opinion a pretty sloppy retcon. How Kubo can be so clever with some things like those minute hints he drops into the story literally years before they ever become relevant and then go and get into trouble like this TZ plot mess is bewildering. Here is exactly what I am talking about:
-Kubo goes extreme makeover fangirl edition for TZ in Ichigo's inner world.
-TZ merges with hollow(we don't know it's also shinigami power at this time) before Ichigo loses powers.
-Fast forward to now, OMZ is revealed to be Ichigo's quincy power representation NOT Zangetsu, oh and btw, he has been suppressing most of Zangetsu's real power and only been the primary source of Ichigo's power this whole time.
-But wait, how does that affect TZ, bankai mode and Masked mode? was he the primary source then? Was he also masquerading as TZ as well? But wait, TZ was giving the hollow more control over Ichigo(before he tamed it). And hold on, if TZ was really OMZ and OMZ was using his quincy power as Ichigo's main source of power that whole time, then shouldn't his quincy side already have been merged with the hollow and thus shinigami powers from back before FGT thus rendering the whole last chapter as moot? And if OMZ wasn't TZ then OMZ wasn't REALLY in full control and suppressing most of Zangetsu's power and he definitely wasn't the primary source of power in bankai and so OMZ was pretty much just full of shit in that last chapter. In which case, I don't see how Ichigo could get much stronger as having full access to a REAL bankai definitely means he has access to most of Zangetsu's power as shikai power can't nearly match bankai.
...see where I'm going? It's just so... messy.