This is almost textbook countersignalling. The same as: - Signalling: I dress more formally than everyone else to make up for the fact I'm less professional in other ways - No signalling: I dress like everyone else because I am like everyone else - Countersignalling: I wear ratty old clothes with holes in them, and nobody will dare to question it because I'm the important one here
The author does not actually know why people write with poor spelling/grammar nor truly how others would interpret them writing with with poor spelling/grammar.
They have a guess, but there are any number of alternate reasons why someone might write poorly. They could be technologically illiterate, fat fingered, easily frustrated, mirroring their children, need glasses, careless or any other number of reasons. The only way to find out is to ask.
Engaging in mind reading is fraught with danger. You're more likely to project your own own mood, stereotypes, behavior or beliefs on to others than actually guess what someone's thinking.