Well the way I interpreted it was that Ichigo was on a different level of power than Aizen so Aizen nor anyone else could sense his resitau. Not that he changed it all to physical str like Aizen thought. Aizen thought that because he couldnt comprehend or sense the power that Ichigo achieved. This is the same reason no1 but Ichigo could sense Aizens retiatsus.
With that said its clear that he does not have the power of FGT. It does not take a veteran to realize that.