It's a general statement for those not familiar with mach, TAS, IAS and OAT, and how each factor also limits the altitude capability of the aircraft (ok, TAS isn't going to limit altitude. But it does factor into fuel savings.)

The heavier the aircraft is at the levels at which many trans-continental flights operate will prevent the aircraft from reaching its best L/D ratio at a given altitude. If the aircraft is lighter, it can get 'on the step' which allows it to reach a better speed since the AOA is lowered.

Now in English, it means that the lighter the aircraft is, the faster it can go at a given altitude per pound of fuel used.

In the ultralight example, it is entirely possible to see an ultralight at jet altitudes... if you have the power to get it up there.