Well, this is the team behind the Spectrum Vega which was, you guessed it, an ARM SOC emulating (pretty bloody perfectly) the range of 48/128k Speccies. The ARM is pretty weak though, and the emulation runs on bare metal with not a lot of wiggle room for any further functionality. What a lot of people don't realise though is that it really is a real, new, ZX Spectrum - it's manufactured by SirClive and designed by Dickinson, and using a low-cost ARM is typical Sinclair.
The 64 will very likely be the same - a low-cost ARM running a C64 emulation on bare metal, albeit a very good one. The indiegogo campaign for the Vega went to 150% of the asked funding, and the new Vega+ campaign is sitting at 367% of its funding, having blown through it within a day of going live.
So there's every chance that the 64 will be very popular when its campaign is kicked off.