They say 'estimated'. A way to do that is to look at the disassembly of IE code, assuming an on average constant number of assembly instructions per C statement. I don't think that is a bad assumption, but it does ignore potential differences between compilers.

Also, IE source likely is available through Microsoft's Shared Source Initiative (http://www.microsoft.com/en-us/sharedsource/default.aspx). After all, Microsoft has vehemently argumented that IE is inseparable from the OS. I think it would require some lenient interpretation of that license to use it for this purpose, though.