From our analysis, any codebase that uses TBB is effectively GPL. However, your binaries are exempt via the Runtime Exception, as Intel intends. That said, anyone of your employees or partners that has access to your source code could effectively treat it as GPL, and re-release it under that license -- hence the risk.
Obviously, you can only request the source code if the binary is released under the GPL license. In this case, however, it's not under the GPL because the Runtime Exception is doing its job correctly for the binaries.
If you got access to the source code otherwise, you could legally treat it as GPL for the reasons I outlined above.
That's not how the GPL works. Just because it's GPL doesn't grant you the right to access it. By default, there's no agreement between you and that company. (In fact, that's the reason companies can sell GPL code.)
You only have the right to request access the code it if the license under which you're given the binary is the GPL. If you were not given the binary, then there's no agreement between you and that company, and the fact that the code is GPL doesn't make any difference to you.