No, brute forcing a pin is not supposed to be possible. And yes, it was always possible, but only if you have Apple's secrets. A short PIN is only secure becaus

No, brute forcing a pin is not supposed to be possible. And yes, it was always possible, but only if you have Apple's secrets. A short PIN is only secure because of rate-limiting. Loading a firmware without rate-limiting is effectively hacking the phone. Apple possessed backdoor to phones that have a short PIN. The FBI was simply asking Apple to walk through a door that Apple created (perhaps unwittingly) for themselves. The case had nothing to do with forcing Apple to backdoor their products, which would mean mandated vulnerabilities that Apple is forced to ship phones with. Apple can, and apparently is, designing future phones to not be vulnerable to this flaw. TPM chips, widely used in PCs, are not vulnerable to such an attack because the rate-limiting is done in the TPM hardware itself.