The forum member didn't say. He merely said he'd bought a rifle from an American builder, a semi-custom builder using a name brand barrel, and sent it to his local proofing house where it failed. I suspect it was improperly breeched, not an actual "barrel failure", and my reason to suspect this is I've bought a barrel in the past offered to be delivered with the breech plug in place, and I found it was short of being flush with the rifling, but that's conjecture on my part as it applies to the European failure.
On the other hand the German house is very very precise in their pre-inspection, and if your touch hole is too large, you fail before any further tests are attempted. Could've been as simple as that.
A barrel "failure" is not limited to it bursting.
LD