It was very tongue in cheek. I heard him on the radio talking about hummus, and he will say he likes it a certain way, but accepts other regions make it coarser, with less tahini etc. I can imagine his reaction to an English woman making 'hummus' with no tahini at all, replacing it with olive oil would be an alien concept. But he has to accept, these are the ingredients at hand to us, and this is our culture. I shall be making hummus the Mary Berry way.
However, Mary Berry is not the person to point at if you want to find someone free of hypocrisy. She is very particular about how certain things must be done, and what she terms as success or failure. Not just on a technical level, a lot of what she sets as exacting standards are based on her opinion. She has plenty of her own pet recipies which must be 'just so' and she won't accept any alternatives, as if only she had ever baked a cake 'to perfection', and only 'perfection' would do. This is what she has been known for, for decades! This is what TGBBO is founded on, and how it sets the tension and builds in drama.
So I think this is what Yotam was playing with. Yes, it is a big joke that he can play with any recipe but not hummus. He can't change that aspect of his palate, it is tied up too much with history. He is a very different type of person, very different chef, with different attitude and relationship to food to Mary Berry. The comparison itself is a bit of a joke.