The issue isn't high fees per se, but rather when coin is "mixed", dust from the original tx is leftover in the wallet. If that wallet is marked as "private" and "safe", it is trivial to link them back to the source if the dust is included in future tx's.

This is not theoretical either. Chainalysis has cracked Wasabi's coinjoins in the past. Likely far more times than has been publicised.