Constant with what tolerance? +,- 0.5°C, something like that? And how do you imagine the scene? Pilot only concentraded only on it temperature instrument, not on throttle, flight line, contant altitude etc...Trials say the temperature was 'constant'. It's quite clear IMHO what it means. It means temperature is not rising, and can be maintained. Indefiniately if you like.
Concretly, pilot managed to keep the T° on a certain gap (that amplitude is not precised...) during his (different?) run(s) at flight level. J'm sure he managed to not overheat and even maintained more or less constant the T° during his runs that duration is precised nowhere in the doc, except for engine settings limitation.
You were shown German trials which shown 570 km/h was maintained with the radiator 3/4 closed and with still maintaining proper coolant temperature.
And why do you insist heavily and only on that doc, there are no other different ones?
http://www.spitfireperformance.com/spit1-109espeed.jpg
Why Flugzeughandbuch datas are not OK? How do you explain discrepancy between different soviet, swiss, british, german, french tests?
Regards