Ubuntu and all flavors used to all provide three years of support for LTS.
Ubuntu Desktop, Ubuntu Server, Ubuntu Cloud, and Ubuntu Base had their supported life extended to 5 years, many years ago, but all flavors remained 3 years.
No flavor has reduced length of LTS, it's just the main Ubuntu products had life extended. It applies to packages found in 'main', so as already stated, your Ubuntu-MATE system will have security fixes & updates for the packages from 'main', ie. those used in common with Ubuntu Desktop.
The only change since the prior post you mention, is Kubuntu only has 3 years of support (Ubuntu Kylin had 5 years for 16.04 [only for 16.04], not Kubuntu)
FYI: I'm speaking here mostly from my knowledge via the Ubuntu News Team having been involved there since 2015; but official member since 2017.