I had assumed this was a timing thing - remember Phil didn't play on BB at all, and we know they'd already done a bit of work on THINFS by then. I don't think it was decided that Phil would be a 'proper' band member until a good chunk of the way through the process and then he came in and played on the remaining songs.

I may be talking out of my arse, that's just how I interpreted it from what Jon said about recording and then what Phil posted on social media. It wasn't until Jon said the record was nearly done that Phil started posting photos of recording and it was soon after than he was 'officially announced' as a band member.