FRIEDMAN: "Critically, a decision was made near the start of the war to deploy the Wagner Group..." (your quote)
You: "...I hadn't heard anything new about the Wagner Group until recently when many of them were reportedly killed in a..."

Friedman's point was that deciding to use Wagner was an issue that contributed to Russia's failure in Ukraine.
It goes like this
Putin decides to treat Wagner like regular army. Army doesn't like that (turf issue) and as they're in control of logistics they 'starve' Wagner, particularly on ammo. Wagner does poorly (probably due to 'starvation', but who really knows) They hold Putin responsible when he doesn't remove Greasimov from control - and they march on Moscow.
Anything that rump Wagner is doing in the Sahel is completely to the side of what Friedman had to say.

All that said, I somewhat agree with Peter Zeihan on Putin's motivations for attacking Ukraine. Russia's in demographic collapse - and they're on a plain well suited to mobile armor (or for that matter Mongol horse cavalry). They have to expand far enough to control geographic barriers - and they have to do it while they still have a cohort of men young enough to be in the army.
This made no sense to me at the time as the RF was a nuclear power - and who invades those (easy geography or not)? Now, it's not clear whether they have no working nukes or are just afraid of what would probably happen if they used one. If Putin knew they had no working nukes (possible but unlikely, given his reliance on a rotten-core army) that would have been a definite reason to try to expand to those geographic barriers (which BTW, are on the *other* side of Ukraine).
Just my $0.02...