I saw this page posted on another thread. Be careful trusting the info provided on the site, some of it may be generalizations based on what other people have told them. I suggest people find an alternate way to decode your serial number and get some basic info about your Mac, without giving some random website (this site does not appear to be associated with Apple in any way) the serial number for your Mac.
The "Submit Info" part of the header is where they get the info they use to provide the generalized information they do. For the question you had, they'll take the reports they get and once they see serials that got shipped with ML, they can then say anything before those serials (there's a small part that is a sequential number) did not have ML.
Also, anyone with access to Apple's service tracking system can decode the serial using an official Apple tool that can provide more detailed info.