They don't say it at all, but perhaps the biggest change here is that they're making headings big and bold now. For quite a few years, Apple, Microsoft, and Google (even pre-material) all seemed to be competing on thinnest, smallest, ultralight-est headings.
edit: I stand corrected. Their marketing site and all their videos are (apparently) inconsistent with their guidelines: https://docs.microsoft.com/en-us/windows/uwp/style/typograph...