[Tres Seaver firstname.lastname@example.org]
FWIW, Ned Batchelder's 'coverage.py' does a good job with branch coverage. I haven't seen anything in this discussion which indicates that binding expressions will change that at all.
I don't think you missed anything relevant either ;-) Binding operators are exactly as irrelevant to control-flow analyzers as, e.g., introducing a floor division operator (//) was.
Data-flow analyzers (if there are any for Python) are a different story, since they need to be aware of all (re)binding operations - although at the byte code level, all such sites remain equally apparent (no new flavor of "store" operation is added by this PEP).