Yes, good point MB. I thought the same. It's worthwhile reading the discussion in the Comments section where he answers several questions taking him to task on that choice. Fuel type was a factor, he required Jet-A1 which he apparently couldn't get at the time at the usual stops across the Nullabor. As you say, he had Forrest as an alternate. But even so, the coastal route, as he says himself, would have added just another 40nm to his total. And, as things turned out, he could have made it with legal reserves that way but I think on his original plan even that bit extra might have put him close to his legal limit (especially if the winds turned).
Anyway, he discusses all that very openly and helpfully in the Comments. He was, of course, set up for ditching (as he explains). As it happened, he was always in VHF contact, and had the back up of talking to airliners on Guard.
Of course, a lot of all that is out of the RAAus league and yet, not that far out. Some of the mid-flight surprises he faced - and shared - could happen to any of us and for me it was good to see how a professional pilot thinks through unexpected plan changes. And it's only in this kind of video that we get to see the pilot's problem solving in action.
Heck, his videos about flying the Citation Mustang and the DA62 are even further beyond my aeronautical ken, but (call me a tragic) I find 'em fascinating. ;- )