What sources, methods, capabilities or other sensitive information would be compromised by identifying the "advanced threat" as, for instance, an Su-27S, an Su-30MK, or an Su-35S? None at all.
However, the threat level that each one presents is very different. The first-gen aircraft had analog RWR and were very dependent on ground control. The MK can carry jamming systems that can give an AIM-120 a few problems. The Su-35S has digital passive EW, integrated jamming and RCS reduction. That doesn't mean it can't be detected but it complicates LPI detection and tracking.
You may think those issues are nit-picking, but you'd be wrong.