M Clubs are supposed to be open 24/7, including weekends. Most Executive Lounges are open Mon-Thu from ~6am through 10pm, for breakfast on Fri, and for evening service on Sun. I consider the significant difference in hours/availability to be material. Most, but not all, M Clubs are located on the lobby level, whereas most Executive Lounges are not. Some M Clubs provide breakfast by allowing access to the full restaurant buffet, which is considerably more comprehensive than the typical Executive Lounge breakfast; I am not aware of any Executive Lounges that provide breakfast in that manner, but it is certainly possible that they exist.

Marriott must feel somewhat strongly about the new M Club model, as they have offered to subsidize the relocation of Executive Lounges to lobby-level M Clubs for some properties.
I thought that correcting the misinformation you posted was helpful. YMMV.