In DOS you literally started windows as an application (not MSDOS 1.0). Windows was originally a visual file manager in MSDOS long before it was short hand for an OS/and full window manager we know today.
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.