Are you sure you didn't sign up for two accounts with two different users?
I earlier wondered about that so I tried logging in using the alt gmail and was taken to a screen requesting I accept their access to info agreement, as if I were registering. I did not proceed.
So strong implication is that being signed up under that account was not my issue (though it's not 100% because maybe terms of usage had changed, or due to a long time of inactivity, system requires a re-confirmation procedure).
Could you try hitting the unsubscribe link in the email you receive?
Now that's such a good idea, I expose myself as a doofus.
The most recent email (to secondary account) had two links at the bottom:
First I tried the alter settings link:
Interesting, huh?
Then I tried unsubscribe link:
I guess we'll see if that eliminates the nuisance emails.
The existence of the problem however remains mysterious.
...
Now, after typing all the above, I notice the link for the alter settings action is simply
https://<SiteThatUsesNodeBB>/user/<AlternGmailAcct>/settings
which prods me to try the link with the trailing /settings removed, and I see it's valid, and it shows:
Well I know from my notes that I was learning to access the site on that date, so apparently I did inadvertently activate that alternate account at that time.
Thank you, PitaJ. I should have been able to solve this without your help.