I think that the reason for decreased depression associated with getting up earlier is that (Western) society is designed by early birds so, getting up earlier allows you to fit in more readily, which helps with self-esteem and thus takes away several routes to depression.

If society was more open to flexible work hours (which is a probable side effect of more people working from home) and making life easier for those with a predisposition to getting up later, I wouldn't be surprised to find that the protective effects of getting up earlier are diminished or, rather, that the likelihood of depression in the first place are diminished.