From the C-172L manual:
If flap settings greater than 20 [degrees] are used in sideslips with full rudder deflection, some elevator oscillation may be noted at normal approach speeds. However this does not affect control of the aircraft
Now if the crosswind is strong enough to call for full rudder, there's likely enough mechanical turbulence that you won't notice. I suspect the certification pilots picked up on this behavior at altitude.
After full flaps are applied you may still want to steepen the approach. Sideslip is the tool to use.