Not really - sounds like the aircraft was operated within its limitations.
Since the supercritical wing prevents the onset of mach tuck when the aircraft is operated within its limitations and the "box" spells out those limitations, coffin corner is a bit of a stretch.

I had a jump seater once panic and yell 'FLAPS, FLAPS!!!' just prior to landing. Turns out he was unfamiliar with our aircraft and the deck angle wigged him out. But he was mainline, and we were not so maybe he knew better?