ISTR reading (probably in Ian Beattie's article on the blue pullmans, so IDK if it is any more accurate than his drawings sometimes were) that the issue was about the difference in pay and conditions between BR catering staff and Pullman (UK) staff, even though Pullman had been nationalised in 1948 and had been a BR subsidiary before being fully absorbed. I can't remember ever reading which way round the difference was.
I get the impression (as an outsider) that in part they are just trying to push up wages and conditions as high as possible before the inevitable redundancies and de-skilling - get while the getting is good, and make sure the baseline for negotiating the redundancy cheque is as big as possible when it comes. It's not great for the industry, but there's no way the industry as currently structured can guarantee jobs (let alone career progression) for many RMT members if they just accept the changes, so I can hardly blame them for that strategy. Paying them off in a lump sum would probably be the best available "whole railway" solution, and might be cheaper for the treasury than the disruption caused by fighting RMT on the Southern, but it wouldn't suit the incumbent operator (who'd prefer to pay a bit and then pass the problem on to their successors) or the government (for obvious political reasons).