Thanks Kj2. That's what I had read too at first but obviously I also wonder how other manufacturers like ZL, ArmyTek, Thrunite, FourSevens, and even Nitecore have moonlight modes on some or even all their lights. Another explanation I have read is that a boost circuit is cheaper than a buck-boost circuit.
My understanding of electronics is poor so I can't say what are the reasons but having read reviews with runtimes of FourSevens lights, I doubt efficiency is the reason why Fenix don't use moonlight.
Going back to the HL25: on the Fenix website it says the output sequence is High - Low - Medium and there is no mention of mode memory. I really hope the light doesn't come on on High all the time as IMHO it wouldn't be very useful on headlamp.