The statement is true in principle, especially given that Monero currently has an issue and its anonymity can be reduced to just four decoys; however, it fails to take into account that while the privacy of CoinJoin transactions decreases over time, the privacy of Monero coins increases over time.

>200 input coinjoins with a ~90% remix rate: https://petsymposium.org/popets/2026/popets-2026-0061.pdf

The privacy of coinjoin transactions increases over time: Sibling outputs remix as inputs in future rounds, so the anonymity set of the pool is continuously expanding.