Average. It’s just an average. I haven’t verified whether the number is accurate (and often it’s probably debatable what qualifies as an empire and at what point it fell) but some empires lasting way longer does nothing to disprove 250 years being the average lifespan.
The second part of what you said is still entirely correct of course, that number has no real predictive capabilities for the collapse of the USA.
Both can be true. The major US parties are both rotten to the core and as such the leadership of the democrats has much of the responsibility for this, but people not voting are still fascism enablers, albeit to a lesser degree than those voting for trump for non-fascism reasons.
Voting third party is one thing, it’s the only way outside of revolution you’re going to escape from the established two parties. Not voting only signals apathy and the only explicit statement is “I’m ok with whoever wins, including the raping racist hitler v2”.