Amash was outspoken against Trump during the impeachment hearings but in today's political world, being a centrist or a moderate doesn't cut it anymore which was abundantly clear from Amy Klobuchar, Tulsi Gabbard, and Andrew Yang's presidential runs. He's a fiscal conservative and I agree with a lot of his policies. He might be able to win over a few of the moderate democrats but there aren't too many voters on the Republican side that would disapprove of the job Trump's doing enough to make them switch over party lines. Amash might be a liability if he runs as a libertarian for the Presidency because people on the fence tend to vote for the incumbent anyways instead of the new candidate from the other party. And let's be honest, Joe Biden is not someone that is a strong candidate to take on Trump so he isn't one to convince people to jump ship.
I doubt he really thinks he can win the election but if he is actually on the ballot and if he can get a few angry tweets lobbed in his direction... who knows, maybe he gets 10 thousand votes in some backwater state and flips the result.
I don't think he can be called a centrist. He used to be hard-right, which bizarrely these days may seem slightly to the left of whatever the Republican party has become but it's not really center. So I doubt he'll sway any Democrats, particularly Bernie bros most likely to dislike Biden.
I should clarify on what I mean -- Being a centrist as in not ideologically but politically and not subscribing to a political party. I used the term inaccurately there.