GitHub more complex than todo-list-on-steroids app so a platform change would not make any real sense. MS today may still have some of its old habits but they do seem to have purged a lot of the "not invented here" problem that caused much embarrassment when the first attempts to migrate HotMail over to MS technologies failed. It also has pretty good integration with relevant MS tools (VS & VS.code, etc.).
I expect to see them moving the base infrastructure over to Azure, but non-MS technologies are well enough supported on the platform so that won't require any notable changes to the main codebase of the product itself (though perhaps some rework of the deployment processes to make them more optimal for their new target network?). These days they care a lot more about what runs on Azure than what is written using .Net and even what runs on Windows, and are comfortable releasing their own code using other tech (VS.code being based on Electron being the first example that springs to mind). They'd prefer you used an MS stack from top to bottom of course, but they are more than happy for projects to use other components in/on Azure.
It'll be interesting to see how they would position it alongside TFS, as there is a lot of overlap between the two products. My guess is they'd keep pushing TFS for people who are completely MS shops and GH for people with more varied stacks.
(self reply as it is too late to edit)
As pointed out in another location I post: Electron was created at GitHub and they are its primary maintainer which may have had some bearing on the decision, and a wider effect as it could touch many other projects. Though as Electron is open source there is always the fork option if the community doesn't like the direction MS go with it.
Looks like their will be two sets of automatic posts on news of any project that used Electron: those bemoaning its use because it is Electron and those bemoaning its use because MS!