Hrm, higher than I'd thought.

I'll note it's a manufacturer claim (e.g., "value never to be exceeded" --- most optimal possibly conditions likely).

What's the significance of LHV in this context?

Lower Heating Value, which ignores the latent heat of vaporization of the water produced by the combustion when computing the energy content of the fuel. Higher Heating Value, which includes that, would give a lower efficiency.

Would LHV calculations discount, say, condensing gas furnaces, which recapture most of the latent heat of fusion of water vapour in exhaust?

Otherwise, what's a typical percentage difference between LHV and HHV efficiencies?

Right, condensing furnaces recover the heat of evaporation (not fusion).

https://en.wikipedia.org/wiki/Heat_of_combustion

The difference for natural gas is about 10%.

Thanks.