Doesn't mean much really. At least not for the people outside that secretive little circle that gets access to source code.
It depends how well MS manages the engagement really. If say, Sterling Archer shows up and grabs the code and runs for it, what are those microsoft pussies going to do?
What guarantee do the EU officials have that the source they inspect is full and complete and will match what vendors provide? Not much unless they are looking at a specific area.
Personally, I think its about MS trying to restore legitimacy to Windows as an OS in light of what our NSA has been up to.