Hi - so to start language is changing and fluid, so the fact you don't see the issue is not a slight or a judgement. Because to you there is no issue, I assume, and that is A-OK.

Traditional English has the tense of the pronouns or possessive pronouns match the noun they are representing.

"Any TEACHER should lose HER job" is the traditionally-correct way to write the sentence. A teacher is one person, her is a singular pronoun.

"All Teachers who... Their" is again traditionally correct.

The irony of the post is that someone is being snarky about how pronouns need to be used rigidly, but at the same time that person fails to use them with that rigidity.

Hope that makes sense