They certainly used to use a third party catering team. My experience was that they were, shall we say, 'disorganised' when I did dining trips with them. On one they actually forgot to serve our breakfast and it took repeated prompting until it actually appeared, after a bit of a mad panic in the kitchen.
However, I believe they have recently changed suppliers and I have no experience of the new one, having only done tours in Standard since. I'd be interested to hear if it's improved.