First of all, which JCM 2000 is that amp. JCM number are just like a series designation. The other numbers might be a chassis number like 2202 or TSL-100.
That is a bit weird to have the OT go bad like that, espically if you weren't playing all that loud at the time. I think the jury will be out on that one for a while.
As far as replacing tubes with matched set not needing biasing, he's got a problem there. The Matched set only means tjhat all four power tubes will be drawing the same current at a specific bias voltage setting. That could be 100ma for one set (which would be low for 4 tubes) or 220ma for another set (which would be too high for 4).
Either way, the bias being off doesn't ususally take the OT out. If it is of the TSL or DSL variety, there is a common anti-oscillation disc capacitor that shorts & could mimmic a bad OT to an unexperienced tech...so I'd be careful at accepting this 1st diagnosis. get a second opinion from a tech that at least understands how biasing works!