Because the post is just rambling without a clear intent or direction. Why do you need "oversell protection" if you have "transactions". Isn't the whole point of a "transaction" that it handles concurrency and disk failures?
Oversell happens because Shopify doesn't decrement inventory until payment is confirmed. And they apparently would rather die than change that invariant.
I could see a historical moment where Shopify, sans that invariant, massively fucked up inventory counts from uncompleted transactions... then had to unwind all that when a bunch failed to clear payment.
Ergo, now there's an invariant.