My understanding is that anuerysm posting is when, even if you account for spelling mistakes and auto correct errors, the text is still indecipherable, as if the person has dysphasia, or is actively having an anuerysm, resulting in grammaticaly unparseable, meaningless sentences, where no salient ‘objective’ of the utterance can be determined.

(Different from schizo posting, where the sentences usually are mostly parseable as having actual meaning, its just that whats being said is completely insane.)