Author here; I agree this is a valid objection and that one should be very cautious when extrapolating cause-effect relationships. Systems like these also often have multiple embedded feedback loops in them, which can make it impossible to identify a single cause once the flywheel gets going.

It's when you combine the USDT/BTC correlation with the available evidence for Tether's unbacked issuance that the problem becomes clearer, in my view. When issuance is unbacked, it can be decoupled from real demand — and that's a degree of freedom that allows Tethers to be injected arbitrarily into the system. Coinbase's stablecoin is backed by audited reserves, so USDC is constrained by demand — making it more plausible that USDT is the causal factor rather than USDC.