You missed the key point of the OP: you only get 70% of the gross revenue (units * price), but when a user requests a refund, you the app developer have to pay out of pocket to return 100% of the price back to the user (Apple doesn't refund the 30% it took off the top). So you are losing 30% on every refund, and in the extreme case, if every sale is refunded (i.e. from a malicious online viral thing directed against you), you can lose unlimited money scaling linearly with the number of sales (again, if every sale is refunded or close to it).

EDIT: ofc in the Twitter thread, there's few disputing that if this is actually true, or if maybe Apple does different things in different markets, etc. so everything with a grain of salt I suppose