Yes, you are correct.
They are saying, that Firefox is better than Omniweb and Safari, because otherwise "hundreds of millions" (that's what one of them said at one time) wouldn't have downloaded it. And since one way of testing whether an argument holds water is to adhere it to similar situtians but see how far it can go, IE and Windows _must be better than anything else out there. That's the McDonald's-argument in a nutshell. Needless to say, it's invalid.
Further, no matter what their arguments are, and whether they actually relate to download numbers, they constantly go "Well, I'm right, I have the dl-numbers to prove it". Again, it's invalid: One could say, "The Fox-icon best App.-icon ever. The DL-numbers prove it" which, frankly, is nonsense just like saying "FF is the best browser in the world. The DL-numbers prove it".
The above is my whole beef with this discussion.