Haven’t had time to dig through it fully but I skimmed through it. Some useful pointers in there. The thing with all these usability pointers is, most people would just take them and use it as it is without questioning whether it’s the right thing to do for their particular case. I feel like they should be taken with a grain of salt
Mobile app users must be presented with multiple sign-in options.
Sure, in most cases. But if you’re building a brand and don’t want to share screenspace with competitors, then maybe that’s a bad idea. Here’s an article from mailchimp and why they chose not to do that