The quality of a protein is measured using PDCAAS (Protein digestibility corrected amino acid score). It's a score between 0 and 1 that measures the quality of a protein as a function of digestibility and how well it meets the human amino acid requirements.

It is indeed correct that both lentils and chickpeas (which the original comment you replied to was talking about) have a much lower PDCAAS value of around 0.70. Data on beef varies, but it is generally considered to be a complete protein with a PDCAAS score above 0.90.

Instead of accusing "people who insist on eating animals" of lacking scientific knowledge, it would have been much more helpful to point out that the highest quality proteins on the PDCAAS scale are almost universally vegetarian or vegan: eggs, milk, soy, and mycoprotein all have higher scores than beef, chicken, or pork.