I wonder if they're strictly emulating just the digital bits of the original hardware.

I mean, the reason we have so many different VSTs for the same devices is that emulating the digital components is the "easy", so to speak, part, while the truly complex and really differentiating part is the software modeling of the analog components - which also explains the difference in price between a free VST replica of the Yamaha DX7 and the circa 199€ Native Instruments and Arturia replicas.