Correct. It's not about the complexity of the code, but rather about trying to enforce standards across the board to make it more consistent for the users, rather than the modders' themselves.
Most of the Launcher's users are not on the forums, nor do they even know it exists, so I have to keep that in mind. If I allow modders to come up with their own unique download methods all the time, and code for them, then that breaks any kind of standardization.
For example, if someone hosts their mod on a wonky site, such as dropbox or media fire, and it expires, then suddenly that mod is not playable in the mod launcher. It'd reflect poorly on the modder themselves, rather than the mod launcher, as other mods would continue to work. That mod would likely not get a second chance from that particular user.
Now, a modder can post a new link on the forums, apologizing for it being wonky, and that it's fixed. But 95% of the mod launcher's users will never see it, and likely won't re-try the mod.
If github goes down, then none of the mods work, and they'll reach out for support, as something is obviously very wrong. Or they'll simply re-try the next day.