That's fine to say it is so on paper however the legal limit is not respected with several common household brands testing at 8-9% in last results I can find. They're incentivized to pad the product with anti caking agents to reduce cost, and it is essentially unenforced. Expect this to worsen as FDA is undergoing planned dismantlement.
Please provide a link to back this up. The Eater.com article you linked to elsewhere is from 2016 and refers to a specific enforcement action where a company plead guilty to food adulteration for adding excessive cellulose to parmesan cheese. Not a great example of something being "essentially unenforced."
B) it's legally limited to 4%, not 10%