I'm extremely skeptical of this claim. I have apps in the app store that behave exactly as you've described and Apple has not rejected any of our submissions. One app uses a third party login, one uses a first party login. One allows for registration within the app, the other requires registration outside of the app.

>it's been quite awhile since multiple weeks without a response has been normal

You are correct that typically reviews are very fast. That said, we recently had a new version of an app take 2 weeks (14 days) to get reviewed and rejected due to a similar, but different, issue. My guiddance is always submit very early (4+ weeks), especially when you have a launch date with marketing or other real-world tie-ins.

There are easy ways to work around Hey Calendar's rejection, but only if you submit early and are prepared to do the work. Similar to your suggestion, providing a "Demo Mode" where a user can see what the app does without login easily solves the rejection issue. I can say this with confidence as I've done it in the past with an app that is still on the app store.