Do Biuret test always means proteins presence in a substance? I am a student and I got this answer I cant really say for sure if I understood correctly :

"A substance reacts positively to Biuret's test. ¿ Can you say with 100% accuracy that such substance is a protein? Justify"

Is this correct to say that the substance is a protein?
Is there a chance that the test reacts but the substance is NOT a protein?

Additionally, I am being asked why we are using whole milk instead of skim milk in this test.
Biuret test react with a peptide bond, so nylon can react with.( https://en.wikipedia.org/wiki/Biuret_test, https://twitter.com/jane_essex/status/921359485939134465)