It never made much sense to me. How statistically "an hour" came out, probably relates to overall survivability at the time it was loosely defined, and with improvements to care, it's now clearer how many prior "they didn't survive" might have, given more immediate care.

Richard Holmes worked from the tallies WW1 Surgeons had to complete, documenting how battlefield dressing stations and rear echelon surgery worked in those days and it was pretty awful. But there are remarkable tales of resilience and survival right the way back to Waterloo.

An hour is just a simple, easily remembered value, not a hard rule. If it takes 62 minutes or 58 minutes there's unlikely to be a difference.

The real cleverness is fast triage under duress and especially in mass casualty events where you may go from 1 patient per hour to a dozen in a minute as an entire rifle squad or whatever gets carried in. In those conditions you basically park the unlikely-to-survive round the back, the non lethal wounds off on the side, and try to solve for the hard middle cases: the abdominal injuries for example. And even those are sometimes just "clean up what you can, pack the cavity with a ton of gauze, sedate and get them to hospital in Germany asap"