centrists working with leftists to keep fascist out of the gov?

This was voters siding with left-liberals when faced with the choice of an unpopular centrist incumbent, an increasingly deranged nationalist movement, and a coalition of moderate progressives. Macron’s original stated gamble was to allow National Rally power to govern in order to prove their policies were unpopular. But he miscalculated how unpopular LePen’s movement actually was and handed a plurality to Melenchon instead by mistake.

I’m pretty sure the leftists know the stakes

People routinely vote on vibes. Hence the Obama-Trump voter or the wild swings between Tory and Labour in the UK. I would not bank against a momentary coalition falling apart as soon as Macron is asked to compromise on some pro-business policy or step towards increased at-cost domestic production.

I certainly would not bank on the largely right-wing owned domestic media whipping local French citizens into a hysterical lather over the advent of the Evil French Communists taking a few Parliamentary offices. Public opinion can turn hard right with even a marginal economic downturn if news media is there to scream blame at migrants, (((bad actors))), and brown people loudly enough.