Getting a Boeing PR person and a random Boeing engineer to say “that sounds reasonable” hardly counts as proof. I think I can help debunk this “proof” using only public sources…
The airplane is flight-tested and certified to be safe (flutter-free, able to carry gust loads, controllable, etc.) up to Vd (the maximum dive speed). This speed may be found in the type certification documents that are filed with the FAA.. For the 767-200 at Sea Level, this speed can be found in Type Certificate # A1NM, available from the FAA at the follwing web site:
rgl.faa.gov/Regulatory_and_Guidance_library/rgMakeModel.nsf/0/15302e51a401f11a8625718b00658962/$FILE/A1NM.pdf
If you want to find it yourself, just do a Google search for “Type certificate A1NM Vd”. This document clearly states that the Vd for a 767-200 is 420KCAS (knots calibrated airspeed). This is the equivalent of 420*1.1516 = 484 mph (calibrated).
At sea level on a standard day, this would also be the true airspeed. As it is, the corrections are small. Now you need to compensate for compressibility to convert to equivalent airspeed. At sea level on a standard day, calibrated airspeed and equivalent airspeed are equal. At 700ft and 420KCAS, we’ll have a downward correction about one knot – say 419KEAS.
Next we correct to true airspeed. Assuming a standard day, the air density at 700 feet is 97.9806% of that at Sea Level. V = Ve /(.979806)^.5 = 419 / 1.01025 = 423.3 knots. Now lets convert to mph. 423.3*1.1516 = 487.5 mph (true).
Remember, the airplane has actually been flight-tested at various altitudes (all the way down to near-sea-level) to establish these values, and in fact, it is standard industry practice to go a bit faster during the flight test and set these Vd values slightly to the conservative side. So, odds are that the 767 was flight-tested to about 500mph in a dive near sea-level.