Black beans contain 9g of protein per 100g. But it's contained within fiber so less than that, if at all, is available to us. All other pulses and beans and such contain about the same % protein, also contained within fiber.

Nuts are 100% indigestible, they go straight through, we can see them intact.

Seeds are indigestible as is, that's why we process them. Corn is famous for going through intact. At most, some of its sugar is released or made available during cooking, we can taste that. Wheat is famous for its toxic gluten, a protein incidentally.

I could go through a lengthy list here, but the bulk of stuff we get from all that is starch and indigestible fiber, not protein, especially not fat in any significant quantity. In other words, none of it is LC friendly.

For the protein supplements, if it comes from soy for example, it must be extensively processed at the industrial scale using at least some chemical agent. Doing a quick search, I found that soy protein isolates was used solely for industrial purposes, before the 50's. This translates to "not for human consumption". The same industrial processes are used, albeit with sanitary precautions, to produce soy protein isolates for human consumption as a food. Ya so they used to make dirty soy glue, now they make clean soy glue, and we eat the soy glue.

If I'm not mistaken, the least processed (yet within the processed category) protein source is dried meat, i.e. jerky. We can digest that fully and access all of the protein therein, which consists a very high proportion of the mass (90%+), never mind the extensive nutrition profile otherwise.

So, however that guy figured out the "priority for sources of protein", I'm 100% sure he's an idjit. I simply cannot find a single compelling argument in anything he said.