Ah, that makes a lot of sense! The tendrils of my own peas were very fibrous and not sweet at all. I'll try some proper hypertendril ones one day.
I read Sörens entry about the pea soup experiment and I read the link to the original instructions. It's interesting, ansjovis in pea soup ? that is new to me. But Denmark uses more fish than we do in our forestry area far from the sea.
The issue with old cooking instructions is that they rarely are specific about the exact ingredients. Because back then everyone knew exactly what kind of pea to use. A 'no need to state the obvious' kind of thing. This one is not different, it just says grey peas and not what type of grey peas. There used to be many more different types than we have left today. I have a feeling that the dimpled ones from the blog entry are not the type which was used originally. I am pretty sure that they used round grey peas. The deeper the dimples/wrinkles, the harder they will stay despite long cooking time.
But the majority of the old varieties that got saved from going extinct are of the dimpled kind that is sweet enough for fresh consumption. The starchy grey peas became less popular when garden peas arrived. Nowadays most people don't even grow peas for drying at all. It's all about snow peas and snap peas nowadays, shelling and drying types are getting rare.