What people? When? Where? The poet Catullus (late Republic) mentioned urine as a teeth whitener in one of his work

(source: https://en.m.wikisource.org/wiki/Translation:Catullus_39)

However, he mentions it in what is essentially a diss track of Egnatius. So I’ll say that this claim on its own carries little weight.

Urine has been historically used as a textile and leather softener throughout much of human history (at least in western Europe, I can’t speak for the other regions). It was also used as a color fixative.

(Source: “Ye Shall Have it Cleane: Textile Cleaning Techniques in Renaissance Europe”, article by Drea Leed in “Medieval Clothing and Textiles vol. 2”, edited by Robin Netherton, Gale R. Owen-Crocker, 2006)

While its use in the textile industry is well-attested from the Roman period to the Renaissance (and possibly afterwards, I specialized in Late Antiquity, I wouldn’t know), I cannot say with certainty that it was used as a teeth cleaner. The ammonia in urine makes it certainly possible, but I would need your question to be more specific to know where to start looking.