The "mechanisms which have allowed them to clamp down on malicious financial activity" are their payment gateways, the rules surrounding their use use, the ability to perform oversight over app payment activity, and the threat (whether actual or perceived) of total banishment if they are caught—particularly in Apple's case where there's no alternative path to customers.

In the same breath, Google and Apple are also being told that these very same mechanisms are a concern to the ACCC.