The letter claimed exactly the same as the first, namely that if his letter wasn't published, he would be angry.
Now, by the rules I have laid out you would expect that wasn't, the indicative form, would be appropriate because we didn't know if his letter was published. Actually, it is not what we know that matters but what the writer of the second letter knew, but it nets out to the same grammatical issue. If this were a counterfactual conditional, i.e., it was known that the letter wasn't published, then weren't, the past subjunctive form, would be correct. But it isn't a counterfactual, so wasn't is OK here.
However, based on his logic in his answer, wouldn’t the subjunctive be correct only if it was known that the letter was published? I.e.,
If the letter weren’t published, he would be angry. (And the letter was published.)
*If the letter wasn’t published, he would be angry <—- this one wouldn’t work in the case that the letter was published, right?
I acknowledge it’s slightly strange to make a new post asking about someone else’s logic, but I’ve been reading it for a while and it’s bothering me! I would simply have commented on his answer, but I didn’t have the option to do so.
EDIT: I don’t need explanations on when to use which construction...this is literally a post analyzing someone else’s comment that I think has an error in it and it’s driving me crazy because it doesn’t feel like a comment that would have an error in it and yes probably super hair-splitty irrelevant but what is it we’ve all come to do here anyway