"Mach tuck" is old hat for old airplanes. Certification requirements assure that there are no adverse handling issues at high Mach numbers.
Certification also requires that, from M_mmo, the airplane is put into a 7° nose-down dive for 20 seconds. It is not a routine manoeuvre. (K9 is thereby mistaken that "all promises from the manufacturer go out the window" above M_mmo. But more than 20 seconds at more than 7° nose-down is something that will likely be looked at in the tunnel but not in flight test.)
So-called "supercritical" wings are designed such that at operating speeds some of the airflow over the wing is supersonic in some regions. To put it in symbols, M_crit < M_mmo. Many wings desiged nowadays are supercritical, including all Airbuses.
PBL