Parts of a Ubuntu MATE system come with five years of support; as a large percentage of the system is just the Ubuntu base which has 5 years of standard support.
The Ubuntu MATE team provide 3 years for their components (just as all flavors do), but anyone with MOTU credentials (or better) can still SRU security fixes during the five years as the universe or community repository remains open for SRU/fixes for five years; so technically it could be said there is five years if community members step up, but there is only three years of guaranteed support provided by Ubuntu-MATE.
There is a reason for the way you see details broken up in commands like ubuntu-security-status.