Nice to see that Renji didn't recognise Zabimaru at first. This just shows that they are human forms, not the 'real' ones. With the known exception of Zangetsu, offcourse.

Maybe someone else already adressed this, but then I've overlooked it:
It could be a plothole, or something else is going on. The thing is, Ichigo's Zanpakutou is still in his shikai form. If Zangetsu would be removed from it, shouldn't it go back to his normal form?