Prioritising certain protocols has the same effect as throttling. YouTube is very awkward on MWEB any time of the day. MWEB promised that we would not see much shaping after hours, but in reality they have a system which has no room to relax the shaping and it is done around the clock. Occasionally they add more capacity, which gives us a bit of a breather, but this soon disappears. Whether new signups are taking it or MWEB is switching the shapers on and off to deflect criticism is debatable.
Hooray for conspiracy theories. Doesn't sound like you have an axe to grind at all.
Prioritising certain protocols only has "the same effect as throttling" on low priority protocols. Normal priority protocols aren't negatively affected, so it isn't the same as throttling at all.