"The real world is a bitch, and you learn not to make assumptions."
The assumption is yours. Jed was not talking about increased pressure due to the reaction but that due to the gram levels of reactants
Jed said : If enough O2 got in to cause significant heat, the pressure would rise"
He did not say "If the O2 reacted the pressure would rise"
The reactor pressure is monitored to be about 760pa, with only 3 milligrams of deuterium gas
If there were 3 grams of deuterium in the chamber the pressure would be 760,000 pascals or over 7 atmosphere's worth
To get significant heat such as 1- 5 MJ by combustion with O2 you will require to have a much higher mass of D2 /O2 the chamber.
.. gram levels.
As you have stated "Enthalpy of combustion of H2 is 280kJ/mol or 0.14MJ/g" and therefore that of D2 is 0.07MJ/g.
I'll leave it to you to work out the exact numbers ... but the pressure won't be 760 Pa.. or 1000 Pa due to the required gram levels of D2/O2 in the chamber
It will be much much greater than 7600 Pa.
However the pressure is around 760 Pa - 1000 Pa in the chamber throughout the 80 kilosecond test period.
And the minute quantities (3 milligrams of Deuterium) appear to produce megajoules of heat.
Mizuno designed the air calorimetry system to be able to detect down to about 0.2 MJ of heat
but even this smaller amount of heat would require 2.85 grams of D2 to combust with several times that mass of O2
according to your explanation