From experience I can tell you, it isn't.
Since everything in life is a learning opportunity, I hope that your takeaway from this one is to always carry:
1) Extra serp belt
2) Tool set capable of minor repairs, including the appropriate 12pt. socket (what is it, 17mm?) to act on the belt tensioner.
I've had two belt failures in time I've owned my van - appx 100k miles. One was on a road trip to Big Sur, fortunately I've experienced enough serp belt failures in the past (with other vehicles) to ensure that I always carry extras. One of my idler pulleys split in half and turned the serp belt into spaghetti, wrapping it around the fan/water pump pulley. I ALSO carry a spare idler pulley because why not? Last thing I want is to be stranded somewhere in a very preventable situation...
In my experience belts do not just fall off without reason, and when the tensioner fails it tilts towards the front of the van, when bad enough it will "spill" the belt right off. So I'd guess that if you look at your tensioner it's probably separated? In your shoes I would also carefully inspect all of your pulleys to see if there are any other issues present. Check for cracks in the idler pulleys. The alternator pulley is clutched, and when you grab it and spin it then stop its force with your hand the internals should keep spinning for a second or two. I forget which direction but just try both. If you can spin it with you hand and stop it (without taking your hand off) in both directions and it also immediately stops rotation of the internals, then your alternator clutch has failed. Not the issue of your current situation, but once things start happening to the belt driven components they're all relatable. Over time a bad alternator clutch - meant to absorb minute changes in frequency of engine crank output pulses from firing of individual cylinders - will wear on the belt and potentially cause premature failure.
All of this to say, when you have a belt let loose, it's a great time to inspect every single component up front and change/do PM where necessary. I recently had my tensioner fail, it was an INA that lasted about 50k-60K miles. I also had a new thermostat and water pump on the shelf waiting for a good time to install them (van has 280K with unknown history before I bought it, though it seemed well maintained), and it turned out my alternator clutch was bad. So, van got a new belt, water pump, thermostat, alternator, and tensioner. Now it's hopefully good to go for a while.
Perhaps your van will get some front end engine love now too? Everyone's lonely in quarantine, even vans...