For broccoli? No, that’s actually a fairly bioavailable protein source, especially when cooked. Even compared to high protein plant sources like beans or lentils, broccoli’s protein is more bioavailable. It’s just there’s not that much of it, and the “protein per calorie” stat is kinda cherry picked from the fact that broccoli is low carb and low fat, rather than being really high in protein.

Either way, though, the gut microbiomes of herbivores are mainly adapted to draw out more useful carbohydrate, not more useful protein. Cellulose is so plentiful in nature that making use of all those glucose molecules is biologically rewarding.

Plant protein bioavailability is affected by a lot of factors, including whether the food has been chopped up, peeled, cooked, etc. Human chewing and guts (especially in modern humans) are pretty inefficient for extracting nutrients from food in the natural state because we have been cooking and processing food for millions of years.

So yeah. Cows have complicated guts for digesting cellulose and extracting amino acids from chloroplasts, and innate chewing behaviors that mechanically process grass into mush, but we have knives and fire to do more of that stuff outside of our bodies. And cattle pick and choose grasses (including high protein leafy legumes like clover or alfalfa) depending on their own needs, as well.