Here is an over explanation because thatwasthejoke.jpg is now a dead horse:

Yes that was clear. The joke was that the word autistic looks like acoustic, and "acoustic guitar teacher" is a common phrase. I assume many readers, like myself, read it as acoustic the first time, got confused, realized, and subsequently felt dumb. I pointed this amusing coincidence of semantics out with the mini-mean of "you misspelled X," commonly used to call out subtext or satirically feign misunderstanding, as in this case.