This stuff tells you nothing about why
the book is delayed on Amazon. Sure, it's possible
that Macmillan is withholding it. But I doubt they would do that on purpose. For one thing, it would be costly administrative overhead on their part. Amazon frequently screws up ebook listings, and backlogs of getting stuff up are well known. (I've been waiting on several of my books for some time now. The uploading system from publishers is convoluted and not very amenable to fixing problems.)
God knows I have my differences with publishers, and I'm no fan of the agency model. But I do grow weary of the assumption that every time something bad happens, it's the evil publisher's fault. That's supposed to be the author's line.