Gates convinced IBM that he could build one, as he knew about QDOS, and immediately went and bought QDOS as a base to start with. So, yeah, it was a bit of bluster on his part, but he was able to fulfill the contract.
> immediately went and bought QDOS
Important correction: MS did not buy QDOS -- or SCP.
It licensed it.
Some years later, when MS-DOS was a big hit, then it licensed it.
I suspect this was not in error; it was some form of protective gamble.
It wasn't a "base to start with". MS Dos was QDOS. I was a summer intern in the early 1980s with IBM in Kingston, NY, and had access to the source. Gates didn't even bother to remove the SCP copyright notice from the comments, nor the references to QDOS. Too bad "sed" and "awk" weren't available for search and replace back then.