A couple of points.
I thought I had posted on another thread that the the Germans reverted the Polish enlisted men to civilian status - I think that was about 1941, but I’m not positive of the date. Some of those subsequently died of various causes (including execution and allied bombing) but the mass murder of POWs was a Soviet sport. In any event, those who died/were killed after returning to civilian status don’t count as POWs killed.
Your number killed looks like it may include AK and perhaps Jewish resistance forces. (Warsaw was just one of a number of battles fought by the AK) (Note that while the Germans took brutal acts of retaliation against the Warsaw civilians after the uprising there, the surviving AK were treated as POWs.) I’m not sure if it includes Polish communist units that fought with the Red army. It does likely include Polish units that fought with the British. (Monte Cassino, Arnheim, for two notable examples).
"Today's" NY Times report says the number of Poles killed in action during the 1939 war was 50,000 to 70,000.
And somewhere I read the number who escaped to fight for the allies was circa 100,000 -- doubtless many of those survived the war.
So the figure of 400,000 total Polish military deaths must necessarily include large numbers of others.
Yes, including "resistance fighters" as "military" would explain it.
But our particular discussion is the question of whether the Germans necessarily treated POWs "correctly" -- if the conquered country had signed the Third Geneva Conventions (1929)?
Well, I find it hard to believe that a nation which wantonly murdered millions in Geneva-signing countries like Poland would necessarily show strict scruples when it came to treating their POWs.
However, for the moment at least, I've found no hard data to confirm it.