Correct, it wasn't a window, but it also wasn't an emergency door that detached. Alaska has it's 737 Max 9 planes configured with fewer seats than normal, and therefore is only required to have 6 emergency exits instead of the standard 8 for the Max 9. So they took out the extra 2 emergency exit doors and plugged up the space. It was one of those "plugs" that detached.
Interesting. Thanks.
It almost looks like the factory installed a temporary plug that wasn’t structurally attached and was meant to be replaced with a permanent plug later on. It’s shocking how that “plug” blew out without any structural damage where it connects to the fuselage.