Me, before:"The major disadvantage from the Anglo/Duet handstrap is that it locks the fingers"

Bernard:"Not so with the Anglo. It's all down to good technique".

RE:Hm..."good technique" may compensate a dysfunctional tool to some part, for sure. A good craftsman may do fine carvings with an axe but probably better ones with a good knife.
It also depends what "technique" there is - it may have drawbacks in its turn, more below.

Bernard:"If you play with your fingers pointing forwards (your elbows at roughly ninety degrees), then I'd agree, as the weight of the instrument will compress your fingers together.
However, if you have your fingers pointing upwards (your elbows at roughly twenty degrees), you will find that the weight distribution is radically changed, your fingers are freed up and you have no problems".

RE:Yes, not new to me at all. Depends entirely what you are up to for playing.What you are doing is relying on the handbar for control and the handstrap is not so tight, just as you say.This is a common way handling the Anglo for Morris ( is that your own style?) which means rather bouncy rhythmic music with much chording substance in it.I haven't seen any "Irish Anglo" players use this position despite one would expect greater obstacles from "compressed" fingers there - with demands for more elaborate single note fingering - but they rely on stabilizing the instrument upon the knee instead.
Both ways surely present "technique" to deal with the situation which however still means 'using an axe instead of a knife' for the same occupation.

What is ergonomically "wrong" with this high playing position is that the arms are lifted and kept up by static effort, something that always should be avoided .IF playing any concertina standing, the natural relaxed and efficient position is with the instrument low, upper arms near vertical and an open elbow angle, about 120 degrees, but for very energetic pumping a 90 degree angle at the elbow and horisontal forearms might be more efficient.High arms is always more strainful.

And again - whatever technique and position of the arms is used the position of the hand vs the surface (level of the keyboard) is not ideal concerning any traditional concertinas except the "Chemnitzer" style ones with their 'platform' for the hand.