What your describing is outliers. Smurfs destroy the MM rules. IMO fixing that would fix a lot. In this example the MM is seeing Fan (The best) as a worst (-50). But even then Fan loses a lot of matches through a B2GM play through, which suggests the worst player does effect his games more than you realise. Last play though I watched he was hardstuck.
Personally I actually like the shared XP more than the LOL way but it would definately be interesting to know what the game would have been like had this been implimented.
I believe what they are describing is the “feels”. You probably won’t notice that smurf that manages to soak an extra 2 lanes of xp while still making it to the team fight but you will notice the B5 that runs in like Leeroy Jenkins and dies 1 v 5.