Interesting...
ZeroAvia have made quite good progress flying their Dornier 228 demo, and I think they have also flown a Dash 8 300, but they are definitely a long way from getting anything certified for the moment. That being said it definitely seems like a more feasible short term option than the battery conversions, and bespoke new EV aircraft designs.
Flying a Twin Otter from Edinburgh to Southampton seems very odd, regardless of the fuel. That must be a >2hr sector length, on an aircraft with no toilet(?) and no cabin service. This will be up against up to 5 daily Loganair flights, taking 1hr 20 on a considerably more comfy and better equipped E145, with cabin service. Choosing a 356mi (great circle) route when the electric aircraft will have an estimated range of 300mi...
Something like Southampton to Leeds may be a better bet - poor train connection, more feasible distance, currently unserved but has sustained high fares on small aircraft in the past (no guarantee of success of course...).
I would love to be proven wrong, but the mismatch of details with the first press release is not confidence inspiring, and seems a little surprising from a person who has extensive experience and some success in industry (albeit other industries).