Ah I see now, I had to expand a section on that page to see that text.

Yes, it claims that it is okay for informal speech, and I disagree. For me, it sounds really jarring to hear people drop the "ly", and every time I hear someone do that, I subconsciously correct them.

I do not agree with the premise that commonly made errors are "correct" in informal speech, nor that posts full of grammar errors are okay in an informal setting like social media, and the reason is that kids learn how to spell by being exposed to written text, and the majority of that text these days is social media, meaning that they're being constantly exposed to errors, so they never learn how to spell, learn proper grammar, or how to professionally communicate.

There's a very fine line, in my mind, between 'informal', and 'incorrect', and in this case, I think misuse of adverbs falls into the incorrect category.

In other words,It's okay to make mistakes. It's not okay to consistently misspell words and simply declare it to be correct simply because the writer uses language that way.