Hi Andrew -
Back in the days of the Rivendell email list - hosted by Alex Wetmore and moderated by Rivendell employees (most notably Sterling Hada), you could subscribe with any email account. When the email list was disbanded and the Google Group was created, I subscribed with my junk email/Gmail address.
As you can imagine, this caused problems when selling stuff. People would Paypal my junk Gmail address instead of my main email address. Eventually, I tired of this and inquired of list admin Jim if it was possible to subscribe with a non-Gmail address. Jim said there were no rules or regulations requiring a Gmail address.
So I created a Google account with my main email address. I am happy to report several years later, that I see no loss of functionality throughout the Google apps. I would note that I heartily disdain use of any phone. My access of the list (and all Google apps save Chrome and Google maps) is through a desktop computer. And my desktop runs Mac OS 10.12.6 from 2014.
I get the email digest. I can post on the web and/or via email.
I see no downside to accessing the group via a non-Gmail address. I say go for it!
Regards,
Corwin