They should be tight lipped, and many consumer electronic companies go through every measure to avoid leaks. Leaks are good for hype, but bad for sales of current hardware. If a substantial, credible leak got out last year that, say, MS would be releasing the new X-Box in May of 2013, then Christmas sales of existing hardware and games would drastically decline. Illegitimate leaks or difficult to confirm leaks get little play, but if there were major leaks about new hardware, then mainstream outlets would pick it up and more people would be hesitant to buy current gen hardware (or expect deeper discounts). Retailers would try to clear their shelves for the new hardware and be less eager to make a big inventory purchase.
Also, the hardware specifications are also under an NDA for Microsoft
from their other hardware manufacturers. If NVidia is working on a new chip for Microsoft, they don't want the public, investors, and other competitors to know about it until the release is imminent and scheduled. If a leak gets out that somebody is working on a new graphics chip, it will encourage Nvidia's competitors to do something similar, encourage consumers not to buy the current in store models.
Finally, there are purchasing reasons. If leaks come out that Microsoft is developing their hardware that has certain specifications, let's say hypothetically, SSD's, and that every new X-Box is going to come with a 512GB SSD, manufacturers of SSD's or manufacturers of the parts that go into SSD's will expect increased demand and can raise the price of their hardware or materials. This is similar to what Walt Disney did in the 1960s when he started buying up land in central Florida, under fake corporation names and fake buyer names, buying small plots of land. Had "Walt Disney" been on the purchase and sale, land owners would have inflated the price of their land or refused to sell. It's not 1:1, but the philosophy of the secrecy is motivated by the same thing.
Leaks are great for hype, but usually terrible for business. Plus, with a major release like this, Microsoft will be able to drum up enough hype through official marketing releases... So links that can be substantiated really only hurt them. They have good reason to avoid them.
What if your line manager doesn't believe you and fires you without retort? Will they still call the number afterwards?
Microsoft isn't like McDonalds, you don't get fired abruptly with no reason, no paper work, and no explanation. Most managers probably have some idea of the process for developing secret projects... Most of the managers have probably been at the company long enough to expect this and may have been involved in secret projects themselves before becoming managers.
Much of Microsoft is nowadays.
Reason for saying this? I have several friends that work for MS and or used to work for Microsoft -- not in any gaming division or anything relevant to videogames -- and they all like working for MS or liked working there. Just about every division of Microsoft scores very well on GlassDoor, as well.