I can see that. The main reason I went for that is because of the changes to canon itself in this AU.
It's been stated that Mash, with the comparison of Matthew in existence, had an easier time of it - she was more warmly received by the staff of Chaldea, and though I wouldn't go so far as to say she was popular, she still was treated with an amount of respect and compassion. She had Roman and, after the incident with Beryl, Ophelia. Both were attentive and made ample time for her emotional needs. Outside of that were several acquaintances that while she was never close to them, she was never maltreated and had pleasant associations.
Compare Matthew, who had a voice in his head (Morgan), Da Vinci (who does her best but is still insanely busy), and Hinako (who we will refer to as mildly distant tsundere at the time). At that point, Pepe was still working on getting through to him, and Matthew was insanely suspicious of anyone remotely 'human'. He was feared if not outright hated by no small number of people thanks to lashing back at Animusphere, and riding the self-imposed isolation train thanks to Beryl. Enter Fou. A small, non-threatening (visually) animal that had wrecked shop the instant someone stepped to him. Matthew met him first, and immediately latched onto him because... well.
-gestures at the support system he had-
Fou is still attached to Mash, but Matthew's powers and attitude influence him directly. By which I mean, Fou is using Mash and Ritsuka as a barometer to whether humanity has something worth saving/sparing. Matthew plays a role more like Altrouge or Merlin, but with a more emotional component on both sides. Mash is still Fou's favorite human (with Ritsuka as a close second as of late), while Matthew is his... well, his.
tl;dr: Mash and Fou still have roughly the same role to each other, but Fou acknowledges Matthew as his master, so his relationship to each has very different connotations. Narrative changes happened because of how the timeline was altered.
