The above quote is absolutely true.

This is what gun proof is supposed to achieve and how it should be done.....

>>>involve the firing through the barrel of a considerably heavier load than is customary in the shooting field, thereby setting up pressure and stress on barrel and action much in excess of the pressure generated by standard load cartridges. Such pressure should, and is intended to, disclose weakness in guns, whether new or used, for it is preferable that weakness be found at a Proof House rather than in the field, where personal injury may result.<<<

You can throw oil on a (standard pressure) cartridge case all day and all night, and it will never change the pressure of that cartridge. All that the "oiling" achieves is to reduce the grip of the brass case against the chamber wall.
Therefore the barrel and chamber is not being subjected to any more pressure than with a dry case, and this means that the barrel and chamber has not been subjected to any "proofing" whatsoever.

Also, the (standard pressure) "oiled" case, with it's reduced grip against the chamber wall, will not put any more stress on the breeching than what a (standard pressure) case head separation will produce.

The proofing done in the manner in which Bramble's gun was done, does nothing as far as testing the gun "above" whatever pressure Bramble's hunting load produced.