OK here's a related question.
Why would this issue with KU need to be spelled out in the contract for the anthology? Assuming that the anthology is not in KU, and not published on Amazon. (The anthology in my case is only going to get distributed via bookfunnel). Why would there have to be something in the anthology contract specifying that I agree not to publish the story in KU? If I sign a contract that gives a publisher first rights, or whatever rights, doesn't that in itself imply that I can't then go give somebody else first- or non-exclusive rights either, whether to KU or anywhere else - does it need to be spelled out more than just "I agree to give exclusive rights"? The person who'd get into trouble if I broke my KU exclusivity is me, surely, not whoever published the anthology? Or am I wrong about that?
Turn it over the other way.
By an anthology contract specifically stating what each author can and cannot do, you avoid that whole question. There are no assumptions, or possible misunderstandings. Each author knows.
Anywhere the word 'assume' crops up means the contract is too vague. Which also implies the person doing the anthology hasn't thought things out properly.
One should not assume anything where a contract is concerned. Besides, how often would an anthology not be on Amazon, even if not in KU?
The problem is not so much who gets in trouble, but that someone should know what will and will not get you into trouble. That's what contracts are for, spelling things out in minute detail, so there is no doubt.
But the bottom line here is, if you already gave exclusive rights to a story for an anthology, putting it in your own anthology as well breaks those rights, and no matter if in KU or not, you have broken a contract, and can be sued. It's one reason why when you give those rights, the contract specifies the term the rights are given for, and how and when they return to the author. Until then, you cannot do anything with the story. KU is just a red herring in this instance.
If you sign a contract giving someone exclusive rights, then you should not even consider doing something else.