I'm guessing the first. I'm a native speaker and I know this word. I don't use it in speaking (it's a bit if a mouthful), but I use it occasionally in writing.

I'm guessing the first. I'm a native speaker and I know this word. I don't use it in speaking (it's a bit if a mouthful), but I use it occasionally in writing. I think it has better imagery. When I hear "tireless", I think of being tired, but forcing myself to stay awake. When I hear "indefatigable", I think of someone playing sports or going on a long journey and pushing through the physical exhaustion. So yeah, a bit of the first and a bit of the third most likely. Walter Bright is the author of the D language, so I expect him to be intimately aware of the effort required, hence the more vivid imagery.