I suppose they'll be working under a fully trained hospital ICU nurse after a spell of training
AFAIK, a lot (the majority?) already work within the NHS. But they are highly trained and highly regarded anyway, and combat training could also give people the edge in a pandemic situation, it's very similar to a war in many ways.
That said, I'm sure people will do their best but intensivists usually undergo intense, long term additional training to become proficient in advanced and ITU care - many will also have years of experience in the role. It simply won't be possible to train all the staff needed to that sort of level within a short space of time, or for the experienced ones to supervise all the newly trained staff sufficiently.