Yes, I'd looked at the Intercity Stations and the limiting platforms were, shortest listed.LNER can get away with it because they run a limited number of services a day, which don't stop at all stations. There are eight stations on the routes concerned - six of them on Aberdeen-Edinburgh - where the HSTs have to use single door operation. Even 128m units might actually increase that number if end doors are available for passenger use.
Admittedly, one of those is Springfield, which has a grand total of five trains a day, six days a week - none of which are HSTs. But Broughty Ferry and Carnoustie are pretty well used.
For comparison of capacity:
170/4: 18 First, 172 Standard, 4 bikes, 1 wheelchair, 6 luggage stacks, 2 toilets
2+4 HST: 32 First, 208 Standard, 2 bikes, 2 wheelchairs, 8 luggage stacks, 3 toilets
2+5 HST: 32 First, 276 Standard, 2 bikes, 2 wheelchairs, 10 luggage stacks, 4 toilets
5-car 222: 50 First, 190 Standard, 2 bikes, 2 wheelchairs, 6 luggage stacks, 4 toilets
Probably what they have in mind, then - perhaps with vehicle end space in mind to provide the stretch goal of ten spaces.
Interestingly enough, the old HiTrans Turbo Boost study proposed 4-car Class 170 layouts that would almost comply with the requirement, in capacity terms at least.
Edinburgh, avoid 18, but this generally isn't used for Aberdeen trains anyway so no problem with 16 (196m) or 17 (197m) which are the usual ones.
Haymarket; Platform 1 is 185m
Inverkeithing; Platform 1 is 188m
Kirkcaldy; Platform 1 is 193m
Leuchars; Platform 1 is 206m
Dundee; avoid bays, but through platforms are >300m
Arbroath; Platform 2 is 230m
Montrose; Platform 2 is 215m
Stonehaven; Platform 1 is 173m
Aberdeen; Platform 4 is 209m
So Stonehaven Platform 1 can take 7.5 23m vehicles.
Other places they might want to stop Dunfermline (if it becomes Inter8Cities, though Inverkeithing really covers this and would involve re-routing) 165m, Markinch (Glenrothes) 148m, Cupar 182m & Carnoustie at 142m. All of which would take 6 23m vehicles.
Places like Springfield and Broughty Ferry should be served by stoppers, probably Turbostars.
But essentially a 5 car 222 is a slight capacity increase on a 4 car HST and a 6 car 222 a slight capacity increase on a 5 car HST. Slightly shorter overall length, but doors slightly nearer the end.