SDL2 wouldn't be appropriate for what they're trying to do.
SDL2 is more of a drawing and graphics library. You tell it to put a triangle here, it puts a triangle there. But it has no idea what a button should look like - how it should behave, how it should be animated - on Mac, on Windows, on KDE, on Gnome, etc. You could try to painstakingly recreate this look and feel for each platform, but it's a lot of effort, you probably won't get it quite right, you'll make oversights on accessibility and internationalisation, and your hard work will instantly look dated once the platform evolves.
To make native GUIs, you need to talk to the libraries that draw platform-native components, like buttons, for you. But of course each platform works totally differently, and the whole affair is honestly kind of a mess, which is why truly native cross-platform applications tend to be fairly rare in practice. Maintaining five different GUI code bases for all sorts of fringe platforms is not, in most cases, a good use of time. For most apps, either you stick to native and cut less significant platforms, or you abandon native altogether and just use a cross-platform wrapper like Electron.
SDL2 is more of a drawing and graphics library. You tell it to put a triangle here, it puts a triangle there. But it has no idea what a button should look like - how it should behave, how it should be animated - on Mac, on Windows, on KDE, on Gnome, etc. You could try to painstakingly recreate this look and feel for each platform, but it's a lot of effort, you probably won't get it quite right, you'll make oversights on accessibility and internationalisation, and your hard work will instantly look dated once the platform evolves.
To make native GUIs, you need to talk to the libraries that draw platform-native components, like buttons, for you. But of course each platform works totally differently, and the whole affair is honestly kind of a mess, which is why truly native cross-platform applications tend to be fairly rare in practice. Maintaining five different GUI code bases for all sorts of fringe platforms is not, in most cases, a good use of time. For most apps, either you stick to native and cut less significant platforms, or you abandon native altogether and just use a cross-platform wrapper like Electron.