I watched it carefully just now. Kemboi definitely lands on the track-level edge right next to the water on the second water jump.
I think the relevant event is at about 2:20 to 2:23 into the race. I found it really hard to see if he landed out of the water or not when rewatching it multiple times. It also looks like maybe he stepped on the line as they left the water jump and went back onto the track, but again, it is hard to see for sure.
Why didn't they just install a rail? Even D3 colleges have a rail through the steeple chase section. Having no rail in an olympic final is just asking for trouble. It is much easier for an accidental lane violation to happen with cones than a rail. This meet is being run by a bunch of amateurs.