It's a staggering number of DOWNLOADs (or download attempts, depending on how the count was made), not the number of developers.
Apple set it up to get big numbers to impress the simple minded news sites. They put a link to the download on their main pages, instead of only being where actual developers would go.
The result was that a lot of people downloaded it, thinking it was an update for their phone. Or would give them magical developer powers.
Most likely a small fraction were from actual developers, and most of those have not been accepted into the beta program yet and cannot test on an actual device.
Doesn't really matter. Even having 100 to 1,000 actual producing developers would be terrific.