Gotta say, I don't agree with that.
Whenever I've seen midcore teams, it's been more about the time schedule than the skill of the players.
I've been part of and helped plenty of highly skilled teams with laid back schedules that described themselves as Midcore. The first example I was part of was Team Squirtle in Solitude back in FCOB/2.4. It's interesting to note on my logs that there's a gap of 71 days from my last 'hardcore group' kill of A3S to my 'midcore group's' first kill. We likely did a very approximate ~50 hours of progression in that time. That's not very hardcore in my book =s
And yeah, it's gotten pretty wishy-washy as this tangent demonstrates, it's pretty fair to say that an Entropy Gaming's 'Midcore' is still likely rather 'Hardcore' in some people's eyes. It's all rather dependent on your own perspective really.