Barrel is literally no different than using metric volume. It’s exactly as accurate, since both are units of volume. The standard barrel has pressure and temperature references for the contents filling the volume, but the volume unit in that scenario is arbitrary, and could just as well use liters.

The commenter you are responding to is either confused or intentionally misleading. The same volume, the barrel, can be expressed in other units of volume. What matters for the standard unit in oil are the conditions and external parameters. The unit is interchangeable and carries none of that actually important meta.