I got the same notice, but luckily found the e-mail in the SPAM folder.
To add to the second point, the MyNBA 2k18 app on Android does the same shit, you can't log on with the Nintendo account. Not even with the e-mail, just the PSN, Xbox or Steam.
Yeah, they clearly weren't prepared for the Switch version.
I actually tweeted the support earlier asking what you're meant to do to log in as a Switch player. It got ignored, lol.