in fact it's the same thing that surprised me too ...
I had interpreted this jam in the spirit of fun and growth together ...
but maybe I misunderstood ...
Viewing post in but... source code?
Sure, we'd use games made over 3 days, in great part by beginner indie devs for whom it's their first project, to showcase what the engine can do? :) "Best game engine to make prototypes with placeholder art, yay!"
This is a nice demo reel, and I can assure you that none of those games were made in 3 days (and none are open source either, which doesn't prevent us from making a show reel, as their authors are happy to give us recordings to put together in a reel).
The requirement for open sourcing the code is more to create some learning resources for new Godot developers, to see how the more experienced devs develop their projects. And for the less experienced dev, it's an opportunity to get feedback on their code and learn from their peers.