A) the fact that this statement can be made is hella dumb. It's not any less true, but it approaches the event horizon of stupidity itself.
B) It's match-up based. 99% of the Omniverse couldn't gank the Anti-Monitor, but The Flash could, doesn't mean he automatically takes anyone who couldn't defeat the Anti-Monitor. That's just plain incorrect. Hypothetically Scarlet Witch could beat the Anti-Monitor
C) Flash's speed doesn't matter, she is literally godlike in that state, it doesn't matter how fast he can do stuff to her before she has time to react because none of the stuff he can do hurts her in that situation.
D) Her alteration of reality has nothing to do with that, House of M takes place in the same universe as everything else. When House of M is over she still has the power, hence why the mutan Decimation takes place (which is totally the wrong word).
E) Yeah I'm basing my argument on the thing Wanda did one time, you're basing yours pretty much on "Flash beat the Anti-Monitor" and that The Flash is fast.
A) The character is powerful verging on true godhood. Stupid as it may seem, he is leagues above and beyond Scarlet Witch.
B) No, she couldn't. Her range of ability is about one planet, and even that she couldn't control completely. She said no more powers, yet powers still existed. She wanted to make a paradise, and people could still openly interfere. Thats not even remote omnipotence, and in the span of her world she is basically Shuma-gorath but nothing like that matters to a guy who thunderclaps your entire universe out of existence along with the millions of universes surrounding you. The flash beat that guy.
C) She has to activate it, so yes, it does matter.
D) House of M takes place in the same universe as everything else, but thats not to say it takes place in the same plane of existence. Regardless, she isn't going to be doing much of anything without a head to form thoughts with, or kinetic energy to use.
E) Two different leagues. Like saying Batman could beat Darksied because he managed to beat Bane. One is a subsonic, though admittedly strong, member of a weaker whole. The other is an omniversal threat. Also the Flash's speed has beaten far more than just one person, one time. He, along with other Flashes, have messed and mucked around with time as a hobby almost. They outran death itself, they are faster than concepts. I don't know if you realize it, but they are essentially faster than anything Scarlet Witch could do. Everything she does is a force, be it magic or direct manipulation its something that needs to be activated and take effect. Death is a concept, its something thats instant in its entirety. I could exclude the Anti-monitor completely from my argument and it wouldn't change the fact that the Flash caused the Flashpoint by taking a casual jog, and is outrunning the fundamental building blocks of existence while carrying on a conversation with someone else.