As an exercise, pretend you’re a 16th century lord, or just a very theatrical person, and raise your arm with a drooping hand, then gently touch your chest with your fingers splayed and wrist at an angle. Your two fingers in the middle naturally touch first, and stay together once you lay down the whole hand, exactly like in the portraits. It’s not a straining position.
I’m also surprised the paper (?) doesn’t go into simple behavioural explanations for this.