This is default since almost 1.5 years now since NM is the only reasonable choice for inexperienced users to get Wi-Fi up and running in no time.
It's not planned to remove nmtui and IMO there's also no need to document anything and especially NOT how to get rid of network-manager since this will be the first thing inexperienced users will do then just to struggle with all those outdated 'set up Wi-Fi on your Raspberry Pi by fiddling around in some text files' tutorials the Internet is flooded with
If someone for whatever reasons wants to 'disable network manager' simply typing in these three words in either of the two search variants this site provides should be sufficient:
Internal forum variant in the upper right corner: https://forum.armbian.com/search/?&q=disable network manager (as usual not that great results)
'Google site search' next to it: https://www.google.com/search?rls=en&q=disable+network+manager+site%3Aforum.armbian.com&oq=disable+network+manager+site%3Aforum.armbian.com (first hit explains everything)