At any given point in time, there must be at least one combination of Debian Linux LTS and MediaWiki that both parties support for an overlapping period of two years.
This is not possible. The MediaWiki LTS cycle is three years, so 1.5 years into the cycle, there is no MediaWiki version at all which will be supported for two years (the old LTS is supported for 1.5 more years, the next LTS is not released yet, and the non-LTS versions are only supported for one year).
I think what's really meant here is not any point in time but the time of the new MediaWiki release?