the sandals helped his blisters, but also he switched to flat pedals from clipless. That gives you extra freedom to move your foot to find a comfortable position, which can help with some joint pain. Me, I find saddle height is the biggest influence on my knee pain (too low, patella pain, too high, popliteal pain; my experience is from cycling from here in scotland to Corsica and back touring around france, among other tours. I'm a lot older and slower than Lachlan, tho)

normal cycling shoes have a cleat you lock into the pedal with that allow very little movement. sometimes just a few mm of position change is all it takes to relieve a nagging ache and im guessing he found it easier to get comfortable with a shoe he can move around on the pedal.

I've stopped mid ride to adjust my cleats just a smidge when i was feeling discomfort. you don't want to mess around with knee pain especially when riding 12 hours a day