Use of "mentioned" vs. "mentioned that"

Question

"The writer mentioned although it deals with eastern mysticism and sexuality, the content is not erotic." - This is a sentence I am copy editing. I am under the impression that "mentioned" should be followed by "that" in this sentence. Could you please let me know your thoughts on this as well?

When I wrote about this to the person who wrote this sentence, this was their response "I have checked it through several proofreading sites like Grammarly, Paperrater, Scribens, etc. The sentence is correct with or without inserting "that" after "mentioned."

Answer 1

They're wrong. The subordinating conjunction "that" is required in this case. In fact, there's a punctuation issue as well—the sentence should be written as:

The writer mentioned that, although it deals with eastern mysticism and sexuality, the content is not erotic.

Hopefully someone with more technical knowledge of English grammar rules can cite what rule was violated, but I'm 100% sure that it's required in this case (even though it can be omitted in certain contexts).

Just goes to show how little one can depend on Grammarly et al. :)