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.

Integrals are an expression that basically has an opening symbol, and an operation that is written at the end of it that is used also as a closing symbol, looks kinda like:$ {some function of x} dx.

The person basically said "the dx part can be written at the start also, and that would make my so mad :3": $ dx {some function of x}.

This gets their so mad because understandably this makes the notation non-standard and harder to read, also you'd have to use parentheses if the expression doesn't just end at the function.

Note: dollar used instead of integral symbol