An Integral is usually written like ∫ f(x) dx or alternatively as df(x)/dx. Please note that this is just a way to apply the Operation Integration, like + applies the Operation Addition. There is no real Multiplication or Division.

But sometimes you can take a shortcut and treat dx as a multiplied constant. This is technically not correct, but under the right circumstances comes to the same solution as the proper way. This then looks like this ∫ f(y) dy/dx dx = ∫ f(y) dy

Another thing you can do is to move multiplicative constants from inside the Integral to in front of the Integral: ∫ 2f(x) dx = 2 ∫ f(x) dx. (That is always correct btw)

What anon did was combine those two things and basically write ∫ f(x) dx = dx ∫ f(x). Which is nonsensical, but given the above rules not easily disproven.

This is more or less the same tactic used by internet trolls just in a mathy way. Purposefully misinterpreting arguments and information, that cost the other party considerably more energy to discover and rebut. Hence the hate fuck.