This assessment is flawed.

McCree, with his LOUD footsteps and relatively slow movement will have some proplims flanking. (remember this is aimed at beginners)

Reaper is a better Flanker.

Lucio, with a speed buff is more appropriate to speed up a slow team.

Mercy can FLY to high areas as long as a fellow team mate is there (in line of sight).

Agreed. Some of the choices on this chart seem to be backwards…