The techie billionaire announced Sunday he was throwing in the towel on the planning process, saying it was clear to him that EM wasn't serious about duking it out ... evidenced, at least in Mark's eyes, in all the excuses Elon has offered to postpone this thing.

MZ writes, "I think we can all agree Elon isn't serious and it's time to move on. I offered a real date. Dana White offered to make this a legit competition for charity. Elon won't confirm a date, then says he needs surgery, and now asks to do a practice round in my backyard instead."

The backyard thing is new news -- and pretty eyebrow-raising if true. Mark adds, "If Elon ever gets serious about a real date and official event, he knows how to reach me. Otherwise, time to move on. I'm going to focus on competing with people who take the sport seriously."

Mark has kinda been teasing this move for a while now ... consistently refuting Elon's characterization of things, and alleging that the Twitter honcho was stonewalling on purpose.

In fact, it wasn't too long ago that Mark said he wasn't holding his breath for Elon -- and despite EM trying to hype up the fight just this past week (by mentioning the prospect of Italy as a venue), it sounds like Mark's throwing cold water on the whole thing ... calling BS.

It was a cool would-be event while it lasted, we suppose ... and time will tell if Mark pulling the plug like this will force Elon to actually commit -- but if he doesn't, it might be over.