Their tactics including not only threats to delist but also threats to reduce visibility does not make it any better. If those numerous examples aren't crystal clear about the former, here's another quote from Valve (page 18 here):

“We basically see any selling of the game on PC, Steam key or not, as a part of the same shared PC market—so even if you weren’t using Steam keys, we’d just choose to stop selling a game if it was always running discounts of 75% off on one store but 50% off on ours . . . .”

When you say "undercutting the sale", I don't know what you mean. They are talking about developers setting lower prices outside Steam, which Valve obviously sees as a disadvantage to Steam. Your DLC example also does not make sense and I don't see that on the list. For a few of the quotes on the list, the type of parity is marked as content, but the overwhelming majority are related to price parity.