This is a circular argument. “It’s not IC because it evolves readily.” IC is defined by having multiple required components, the removal of any one of which causes the system in question to cease functioning. The criteria say nothing about the rarity of those components, nor whether they can evolve. The hypothesis is that if a system meets the criteria, it cannot evolve. Do I have to break out Black Box and get the actual quote?
Further, the point is that IC is not useful as a design detection criterion precisely because such systems evolve readily. To use that as an exclusion criterion for IC is a ‘heads I win, tails you lose’ argument. Can’t see it evolve? Of course not, it’s IC. See it evolve? Can’t be IC then. See the problem? I’m sure most people reading can.
The reality is that one of the main arguments against IC is that functional sequences actually aren’t rare at all, and there are almost always multiple ways, biochemically, to skin a cat. There’s an ongoing thread on this very thing on Peaceful Science right now.
The question always comes down to “well how you quantitatively define “complex” in a way that allows you to sort the things-that-can-evolve to one side and the things-that-cannot-evolve to the other (if such things exist) to the other?” and the answer, every darn time, is shown to be “you can’t.”