See, here is where I would immediately be skeptical about the article:
enzymes that make the digestion of protein incredibly difficult
Enzymes are protein structures and are therefore very susceptible to heat. (I.e. when you would cook an egg and it turns "white" this is proteins beeing degenerated by heat)
You would NEVER eat raw soy (or other legumes thereof). They should always get cooked thoroughly. And in tofu production, you would also cook the soy milk in the process.
I'm not 100% knowledgeable about these specific proteins, but it sounds like big strawman to me from the getgo.
Sorry for the late reply...
Grains are pretty high in protein content. However, oils (e.g. fried foods), sweets and fruits and often pretty low in protein. If you are eating a "what ever I crave" vegan diet with processed foods mixed in, you will easily land below that threshold.
For example, my wife tracks her food sometimes (for a couple weeks each) and she often lands below her recommended target. She is not eating badly but also not suuuper clean and still, she often does not reach her recommended protein amount.
Bottom line is: You can easily be eating "not enough" protein. However, most of the time it will not lead to big problems. But in the long run, this might get problematic. Every person - vegan or not - has to plan their diets well and think about what they eat at least a little. "Being vegan" is not a magic wand of nutrition. We still need to consider what we eat every day.