Looks like this has been going on for a long time:
On the HRA Website dated 2002 there is a comprehensive overview of TC02 and the practice of milkshaking.
TCO2 Questions and Answers
Research Papers
https://www.harness.org.au/hra/papers/TC02Q-A.HTM
One of the findings in those papers:
For a period of time many believed -on anecdotal grass roots evidence and some academic thought- that
milkshakes would hide/mask the use/detection of other drugs but it could not be proven. Anecdotal evidence eventually became reality in the Meadow Mirage case when after many months of testing, analysts found the drug
etorphine("a positive but light reaction") hidden behind an administration of bicarbonate.(See Appeal Judge JG Garvey’s summary of this case for further information.).