I basically agree with this.
Asking, "What is D&D?" is asking for a genre definition - like asking "What is fantasy?" or "What is pulp noir?" Genre definitions are notoriously squishy around the edges.
A genre is defined by the presence of various standard tropes, and their use in a specific style. Exactly how many such tropes need to be present to make a thing a recognizable member of the genre is a matter for discussion.
I whole-heartedly agree; genres and subgenres overlap each other, and theories about explicitly defining ethem vary wildly from expert to expert. And a single work of fiction (or a RPG) may actually belong to several genres and subgenres.