You are comparing apples to oranges, but perhaps I wasn't clear enough in my post.
Initially, I referred to a "differential compression test", also known as a "leak down" test. This is an inspection procedure to determine relative health of the recip engine. It is not a starting procedure. It is performed as follows:
And for the purposes of this discussion, I will use an aircraft engine as an example.
1) The spark plugs are removed from the cylinders. ( on aircraft engines, only one plug per cylinder is removed )
2) The piston is brought to TDC, in the subject cylinder.
3) A regulated test Gage set, comprised of two 100 psig calibrated gages in series, separated by a fixed orifice, (.040", .25" in length) is plumbed into the open spark plug hole.
4) And here's important detail; The propeller is restrained from turning.
5) Compressed air is applied to the test rig, and the regulator is adjusted to 80 psig, and the leak rate of the subject cylinder is noted on the second gage ( downstream of the orifice ) thus the name, "differential" compression check
Now, If the piston is not right on TDC, or in the dwell range, the air pressure will drive the piston down it's stroke, turning the propeller with great force. ( I'll leave the calculations to you, assume a 5" bore, and a healthy cylinder, say 75 PSIG ) even through a .040" orifice.
If you read my post I referred to a "poorly executed" compression check, which I will expound on. The technician performing the test, either through poor training, lack of attentiveness, or whatever reason, plumbed the compressed air line directly into the spark plug opening. Thus a full 125 PSI of unregulated air, without the restrictive orifice, was directed to the top of a 5" piston that was just slightly past TDC. The engine motored rapidly until the air was removed, the Magneto impulse couplings firing away. Luckily, the other cylinder's plugs were removed, or it is possible, with a few more "ducks lined up" that the engine would have lit off. Also lucky that someone wasn't killed or seriously injured.
And regarding your assertion that the flywheel effect of a 60 lb propeller would not carry the piston back up the stroke, I leave those calculations to you. But remember, as the piston approaches BDC, the exhaust valve opens, relieving the compressed air somewhat & allowing the crank to coast through.
You are correct in your description of the "shotgun shell" starter system. I stand corrected.