One thing about St. Sebastian is that there are dozens and dozens of paintings and sculptures depicting his martyrdom, some crude, but many quite exquisite. I agree with you in this case as in many others, the artist is doing something typical for Renaissance paintings, wherein they simultaneously depict a story from saintly hagiography (or from the bible, or from Classical Mythology) while at the same time showing, and making some commentary upon, the realities of their own day. This was intentional.
The thumb ring always seemed odd to me, hard to imagine. How could your thumb even with a ring / hook be strong enough to hold some of those very powerful bows. But I know they did it. I’d love to try it one day. I’ve looked with longing at some websites that sold recurve bows in the past but never could afford one at the times I was thinking about it.
There are many heroic epics where the hero can draw a bow further back than anyone else, which is another thing I found confusing.
The short powerstroke of the latin style crossbow is a whole nother kettle of fish. They really don’t know why or how those things worked yet (in my opinion). We had a big argument about it (yet again) on MyArmoury recently.
The niche that the English style longbow seemed to have been big, relatively heavy (~ 60 gram) arrows – and long arrows- which could be shot at a good distance, especially as ‘clout shooting’ (area shot) weapons.