I wouldn't expect there to be a hardware SID. The only chip they've mentioned so far is an ARM, and the only custom software they talk about is the front-end. It's reasonable to expect a stock existing software emulator like VICE will be used, and it's not known which SID emulation options will be used or available. I suspect it'll depend on how powerful their chosen chip is, since some other popular ARMs can't reach 100% speed in VICE with the better SID options enabled.
The full sized version is rumored to run the same chip, if not the same board. USB support is only mentioned for keyboard (computer keyboard, not music keyboard ) and flash drives.
I'm not sure how this would be better than VICE on a PC for the chiptune community, as using cross-platform music editors as currently done will have a much faster turnaround. The SID emulation quality might be higher on PC; the best audio fidelity you could expect for the mini is being on par with VICE on PC.