The big Microsoft-IBM alliance gave the world... OS/2 and Windows.
IBM and Microsoft signed an contract, that IBM AT clone computers would ship with Microsoft DOS and they would license (and pay for) every-single-copy of it. The openish IBM PC standard allowed for Microsoft to continually expand into their near monopoly on the Business, and Home PC market.
That agreement literally built Microsoft. When they signed that agreement, Microsoft didn't even own the rights to DOS, or had even made an OS. They just licensed a BASIC compiler.
Neither my memory nor the Wikipedia history suggest any formal IBM role in the development of 'Windows' itself, at least not through Windows 3.11. It wasn't a joint project (though I suppose the closeness of the partnership likely meant source/fix/feature sharing behind the scenes).
Then, IBM was formally cooperating with Microsoft on something they thought would be OS/2. To the extent anything co-developed then wound up in Windows NT, that was never IBM's intent. Again, Windows was the Microsoft-controlled branch, not the partnership product.
That would roughly match the assertion by rational-future about which you were dismissive.