[GeoffR]

Being able to recognise spelling or grammatical mistakes is different from being able to spell or construct sentences or type, just as being able to read a novel is different from being able to write one, or being able to understand a language is different from being able to speak it. Learning to analyse doesn't automatically result in the ability to synthesise.

So I can recognise when my toilet needs fixing and I am entitled to criticise the plumber who does a bad job of fixing it, even though I have no idea how to fix it myself.

(Edit: But it is not fair to criticise someone who doesn't claim to be a professional for not meeting professional standards. So to make my point explicit and avoid misunderstandings, it is fair enough for a reader to complain about spelling mistakes in a professionally published book, even if they can't acheive the same standard of spelling themselves. Pointing out spelling mistakes in the complaint doesn't necessarily invalidate that complaint, not even if it is a complaint about spelling.)