That's my understanding. https://insider.office.com/en-us/handbook/test-features mentions that 'most' new features require an active 365 subscription to receive them, so it does imply that non-365 subscribers may get features, but I would think this unlikely. 365 is sold as being continually developed, while 2019 is a snapshot of feature development that is effectively a frozen point from 365 development.
that said, I actually think I got some new features, because I'm checking here https://insider.office.com/en-us/releasenotes/pc/fast and it's talking about "saving objects as pictures" which is supposedly a new thing in Word version 2001 (January 10, 2020) and I have it now on Word 2019 (version 2002 insider fast)
Not sure if it's an oversight on MS's part as all the documentation I've seen (and your post) seemed to confirm that 2019 is just getting security updates. Would be keen to know if you spot further feature updates as time goes on.