Yeah it was sold out (keep in mind the Wii U was pretty much sold out for a few weeks after its release, including Black Friday). Now, here's the question. What method did you use to determine that Microsoft would have sold about 400,000 more units in the US without supply constraints? Please explain how you came to this figure. Seems to me like you're just making a baseless assumption as to what the demand was.
As for the original X-Box sales, I can't really find anything to corroborate that. Only thing I found was on video game sales wikia, but that's a shipped figure and the supposed reports it linked to are dead links. Do you have any source?
Edit: Also, let's not forget that the original X-Box launched with Halo. X-Box 360 launched with Perfect Dark Zero, Kameo, and a pre Modern Warfare Call of Duty.











