I read it as misinformation--i.e., the legal onus is on Biden to prove that his 80M votes weren't fraudulent; not that Trump would use the military to prevent Biden from taking office. In fact, I had to re-read it several times to even determine what coup-like meaning you were suggesting it contained.

This could be seen as setting the stage for something worse down the line... Come the week before inauguration day, and Biden still hasn't "proven" (whatever proven means to Trump anyway) the 80M votes... what will Trump do? refuse to leave because his stipulation wasn't met?

Right, I now see that this Tweet could be interpreted as a veiled threat; I just think it's far more likely that Trump misunderstands (or is lying about) the law. Especially given his recent remarks that he would leave if the electoral college votes for Biden.

Never take Trump's words at face value - even as president.