I'll mark it down as myth, then
There *is* a good reason to wait until there is a sufficient number of cabinets done - which is that it allows you to perform local-area marketing on Fibre availability without having to turn down 90% of the interested people.
However, we already know the BTOR shoot themselves in the foot every 3 months (with the mass delay of every line), so why would we expect them to make a positive effort on this front?