The textual medium of the internet has allowed for the creation of differing written registers. You may have learned to write in a "standard way" in school that implicitly established itself as "the right way to write." What was left out was an analysis of the socio-linguistic component (probably because you were in 5th or 6th grade and this topic is addressed much later in specialized courses).

Proper or "improper" writing can signal formality and casualness and that sort of casualness is how I read the response. For example, I deliberately pick a register when I consider the audience I'm addressing and use it as a tool to convey side-band information. You may have seen social media posts where nearly every word is suffixed with an emoji. I'd consider this its own register. While I might use all lowercase in a text message or on a discord server, I'd likely only expect to read an emoji-laden message on Facebook or Twitter. Also if it came from me, it would probably look sarcastic as that's not a register I typically use.

Either way, people can do this accidentally, purposefully, or sometimes as a function of the medium. Eg: I can't write in an emoji-laden register here because HN strips emoji. An entire register, simply, and absolutely inaccessible. We probably agree that it doesn't fit the tone of the site, but still, we can imagine how the texture of the site would change if it was allowed and the pros and cons of that decision.