Posted previously by LOMCEVAK in response to a similar question:
It is exceeding 0.9M that results in the uncontrollable pitch down; exceeding 300 KIAS when below 0.9M is fully controllable. The limit is always promulgated as M0.9/300kts but the reasons for the M and IAS limits are different. To put this into the context of display flying, please note that M0.9 at ISA, sea level is 595 kts.
Which differs from your quote. Haven't read the pilot's notes but if that's what they say there seems to be a discrepancy.