Presumably on Mac Chrome does the same thing with plugins as it does on Windows. That is the plugins are run in a separate process to the main browser and communicates via Inter Process Comms.
In which case the browser could be 64bit and the plugins could remain as 32 bit (IPC really doesn't care about it at all AFAIK). The plugins wrapper and the browser are merely sharing data through shared memory, pipes, semaphores, etc. So long as the 64bit browser and 32bit plugin wrapper have the same understanding of the data conveyed between them (which should be very easy) then it would all work just fine.
So killing off '32 bit plugins' is more of a political statement than a technological necessity. Except that on Windows (and I presume Macs also) the plugins wrapper is merely the chrome executable itself spawned with appropriate command line options. They'd have to calve off the plugin wrapper portion of their code into a separate executable to pull the trick I've outlined. It's that extra work that is what they've judged is probably not worth their while doing.