I don't know, spring is still 14 months away. RTD and Moffat's seasons took 9-10 months from filming to airing. Even allowing for pre-production, seems possible to get it on air for spring if they did start over the summer.
Broadchurch has finished filming and will be finishing post-production within a few weeks, and Chibnall has known about this for two years so hopefully has not accepted any other new work. I do wonder how much of the rumour is legit and how much of it is speculation - I read something elsewhere stating that Chibnall will be working on Broadchurch until summer, when it will have finished airing months earlier.