This is the equivalent of saying the $999 you saved for your Macbook didn't actually buy you that Macbook, because once your money entered the bank, it became indistinguishable and inseparable from all the other money in the bank, and who knows which pennies went where.
Well, that's true, and if you insist that your $999 bought your Macbook, you can take precautions to ensure that (at enormous temporal sacrifice). As a practical matter, this doesn't matter to anybody, so we come together as a society and agree that transactions don't need a perfectly isolated series of exchanges, so long as the credits and debits match up.
The same is true for energy.
On the other hand, saying that you basically got your Macbook for free (saved all the money, i.e. ran exclusively on wind) can only work if you didn't spend anything else. Saving 999$ but spending 200 more (i.e. also having coal) and not counting it because you somehow earned more money somewhere else (i.e. exported it) is not what I call proper bookkeeping.
