garmin forums are bad

You misread / misunderstood this:

"User B logs in to connect.garmin.com on the web. Goes to Profile and Account [top right corner] --> Account  Settings --> Account Information --> Change email.

Now the scary part: User B suddenly can see User A's email and is able to change it!"

They are saying that User B logged in - presumably as themselves - and navigated to Profile and Account [top right corner] --> Account  Settings --> Account Information --> Change email, where they saw the email address of User A.

We can absolutely try to determine whether User B is really logging in as User B here, but it would be helpful to understand what's being claimed in the OP in the first place