Some additional info on this issue:
I ran this race. I signed up a few weeks before race day and downloaded the course. The download I have is the 2019 course which I didn't think much of because often courses stay the same from year to year. This course has a fair amount of path running and some fairly long (I'd say between quarter and half mile sections) with few markings. This was mostly fine but there were a couple times especially on the island where I had a creeping doubt if I was on course. I ran solo with no one in sight in front of or behind me as well for context.
Here is the important part: The course map I have where the leaders did not turn shows NO turn. It is my opinion that one good possibility for the mistake was the cyclist was familiar with the old course.
I would also say this was one of two or three spots where I had to think very quickly to make a correct turn. Even though it was marked, it was a surprise turn for me. This was not because I knew the course very well, but because the course went off the road onto the path.
Hopefully this sheds some additional light on the situation. I think if I would have spent lots of time studying the 2019 map that I am looking at right now pre-race, it's fairly possible I would have kept going straight as well.