The sentences in question:
From what I’ve heard from his classmates, he has a knack for explaining the tricky, intricate programming concepts in a very easy-to-understand manner. This skill of his clearly reveals that his own fundamental computer concepts are strong and crystal clear.
There is a green highlight on that bold part, and MS word says "Reflexive Pronoun Use (Consider revising)". However, I don't understand what's wrong at all. Please explain to me what is the mistake here. Thanks you!