I don’t think it’s an incorrect use of the semicolon. Semicolons are out of fashion these days, but using a semicolon to join two related clauses is perfectly fine, as far as semicolon usage goes.
But the verb in the second clause is not the elided verb in the first clause, so it is not linking in that fashion. The two clauses are independent of each other.
It's debatable, but they could be linked by [You're thinking of] wrong root word.
I agree that if you think "Wrong root word. You're thinking of Kafir." is acceptable, so too would be the semicolon usage, which is why I said "arguably incorrect" rather than just "incorrect"
"[That is the] wrong root word; You're thinking of Kafir".
Since in the first clause, it seems like the intention is to point to the previous comment that is the matter at hand, which was actually written not just thought about. However, other readings might be possible.
I don't see the wrong usage there, assuming that "Wrong root word. You're thinking of kaffir." would have been correct. Which it seems to be the case to me since both "Wrong root word." and "You're thinking of kaffir." seem to be correct sentences and juxtaposition is OK as well.
