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It’s just milk protein. The majority of protein powders (aside from vegan proteins, and beef etc) either consist of whey, casein, or a mix of the two. Both include all essential amino acids. They also make up all the protein in milk. So…whoever told you that, has no clue what they’re talking about.
Every protein is first digested to aminoacids (building blocks for proteins) and then absorbed. No matter how it’s processed. End of story. Bonus YT link for further reference
What you’ve heard is incorrect. Humans can indeed absorb protein supplements. The protein molecules themselves are the same as the protein molecules found in whole foods. And in some cases they may be even better absorbed in their isolated form vs. from whole foods.