I just remembered that the Stacey Dooley programme mentioned that Jemma had also had radiotherapy (in addition to the chemotherapy). I understand that her chemotherapy was stopped quite quickly, but that she had a much longer course of radiotherapy.
Given that she hasn't been having regular scans, is it possible that the radiotherapy did in fact help to shrink/halt the progress of some tumours, and the new ones have grown after the radiotherapy was completed? That's the only thing that makes sense to me in the context of what she's said.
Based on her most recent update, I also took it to mean that this new treatment was on the NHS, but that it's largely palliative in nature and will possibly buy her a bit more time. Although I'm sure if there's a way to monetise it, Jemma will find it...