My July 1922 Bradshaw's timetable has, between 7 and 8am, 2 trains to Euston, 5 to Broad Street and 4 Bakerloo, all calling at Watford High Street so all running on the "DC▸ " lines in question rather than long-distance trains on other tracks. If I moved the one hour window it's likely I'd get 12 trains so the claim perhaps surprisingly seems accurate; Euston doesn't come out of it too well but does have other faster options on the main line. Just 4 trains in the same period now, all to Euston.
I wasn't questioning running 12 tph on the DC lines; I'm sure they could have managed twice that. It was the idea that the number of services
from Watford on the fast and slow lines would be increased at the same time that sounded odd.
But it now occurs to me that, even if only two slow line paths per hour were newly available into Euston, the benefit of the earlier non-Euston services would not have been taken yet. For one thing, they were rebuilding Chalk Farm Junction. So it was only be after the grade separation was finished that extra capacity on the fast and slow lines became usable into Euston. And I guess almost everything stopped at Watford.