"mixing coins" has no law or regulation against it. you are correct. but the vocal conversation of wanting to launder value (in any currency) is a AML flag

changing fiat to say poker chips.. and then swapping different casino chips with friends in some basement. is not unlawful. but the act of starting with fiat deposted at casino #1 and ending up with fiat from casino #3+#5 would flag up as a AML flag that a bank seen $10k->#1    and then a few days later #3+#5-> fiat

so although the acto of casino chipping is not regulated/monitored. . the bank(fiat) will show a discrepensy of deposit-withdrawal from different places and ofcourse an investigation would occur. not just for AML. but just to ensure about tax into on the magical $10k the bank account later received that cant be explained. thus treated too simply as new income of $10k. rather than just a refund of an earlier $10k spend

so by admitting it was not a spend of $10k and then a refund of $10k (net zero profit). questions then arise as to how money went into one business but came out of another business