I don't understand

if the coach isn't allowed to type in all chat, why is it available to the coach then, if they can have a slot for coach in the during the match can't they just disable the chat to the coach???

He’s making callouts of what to do I’m pretty sure

Like you said (AB) it will be good if Riot do something like this, Coaches only can chat at timeouts / pauses and it's only at their team they can't open All Chat