You say 'have I been misinformed', it'd help if we knew what you had been informed.
319s have not been permitted on the Chatham main line, but as far as I'm aware that doesn't mean they'd be restricted. There's no real need for them to run through Chatham, so they've not been permitted. FCC's 377s used to run to Gillingham in the peaks, but now only run to Rochester.
Obviously the 319s are permitted from Bromley South to Swanley on the slow, which you could consider a follow on from the Chatham main line. They are also permitted on the reversible Chatham loop and up Chatham loop between Chislehurst Junction and St Mary Cray, if you'd consider that as it.