The difference in this case is that a backdoor can be introduced after the fact. That is, it is possible to load a new version of iOS onto this locked phone with the backdoor in place. Ideally, one should not be able to update a locked phone in such a way as to introduce a backdoor to allow for brute force passcode attempts.
First of all this isn't about brute forcing passcode attempts, it is about bypassing the passcode altogether. If the phone is open and can be made to automatically download and install a software upgrade, then this is possible, even if the storage is encrypted. Also, given that Apple is the phone maker, they can of course be coerced to produce phones with this capability, whether they want it or not.
