No need to sign up with coinbase
This article has misunderstood what DELL is doing. DELL uses coinbase as their payment provider when you choose to pay with bitcoin. This means that DELL tells coinbase it wants a payment worth X USD. Coinbase will then create a bitcoin address, determine how many bitcoins will need to be paid for that amount.
Customer proceeds to transfer bitcoins from his wallet (which *could* be coinbase, but need not be). Coinbase sees an incoming payment and tells DELL the payment was complete. DELL will then get the given X USD on their bank account.