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