My hypothesis is that people expect text that has no obvious signs of being an Internet comment to use the more 'serious' language and this case (an Internet comment that is a bit longer than usual) is being classified wrongly as a result.
It's ridiculous to simultaneously say that a piece of writing devoid of context is being classified in a certain way and to say that it contains that's inappropriate for that classification.
The mere presence of phrases like "I am not making this up" tells you that this piece is not intended to be too serious. To say that it's intended seriously but contains non-serious language is a flat-out contradiction.
It could make sense if it was published in some context, like a serious blog or a news site or something, which implied seriousness. But it's a naked text file on the internet. It doesn't have to take a serious tone.