You can sideload on Nokia 2780 Flip thanks to Affe Null
It has been a while since 2780 is out. Most BananaHackers have been waiting a long time to get their hand on a KaiOS 3.x device and Nokia 2780 is the first one released which is not carrier-locked. Many other community members, like Luxferre seem to prefer waiting till a global 3.x device gets released. This 3.x flip phone is not global but because it's not carrier locked, one could import them from third party stores like Amazon.
I haven't got my hand on a 2780, yet. But according to Affe, no code worked on Nokia 2780 Flip to enable debugging like other Nokia models such as the BananaPhone. And using W2D does not enable ADB. It's very unfortunate. But like most other hackers in the world, Affe was not willing to give up till he gets what he wanted and now we have the weeknd Toolbox created by him.
This toolbox enables users to modify the internal storage of 2780. This let's us side-load third-party apps outside KaiStore as well as modifying the pre-installed System and obviously, the stock apps among them.
Remember that debugging is not yet possible on 2780. And side-loading the apps is not done through the debugging protocol of B2G like the other phones. Thus, this phone is not suitable for app development purposes, yet. Unless, of course, you are willing to do some device hacks to make it so.
If you want to learn more about 2780 and the technical story behind the hack, you can read the wiki page dedicated to this phone on BananaHackers wiki.