All cellulose (wood) contains the same BTU's. It's just that some woods are denser and pack more cellulose into a tighter space. So, we say white oak has more BTU's than red oak. But that implies that we are talking about per cubic foot or cord or some given volume.
Green wood uses up a lot of its BTU's in drying the wood before it can burn.
I don't remember anyone discussing BTU's - rather BTU/cord or BTU/lb - nothing was implied.
FWIW, all cellulose may have the same BTU, but wood is not all cellulose. The cellulose/lignin content is not the same for all woods (lignin is 15-20% for typical hardwoods), and lignin is a higher energy fuel than cellulose. This is probably not particularly important in the larger picture.
Green wood doesn't use up any of its own energy in drying. The total available energy in the wood doesn't materially change as it drys. Dry wood burns better because it doesn't require heat, that would otherwise go to pyrolysis, to drive off the moisture. It also burns more efficiently due largely to the higher temperatures possible without the water and water vapor present.
In most cases, there probably isn't enough difference in BTU/lb to overcome the cost difference. The bottom line when picking [hard]wood for the WFO is #1 - dry, #2 - cost. For all practical purposes, the BTU discussion is meaningless.