That map isn’t accurate from what we’ve seen in the show. At the beginning of episode 3 (River), they are looking at a river discussing what to do. I don’t think we were told specifically which river it was, but there were some clues it was the Red. Shea wanted to go east to the Denison ferry which is on the Red. He argued that if they went west, they would have to cross the Brazos. The way it was said strongly suggested they weren’t looking at the Brazos as the conversation was happening. So the Reddit map you posted doesn’t show them going to the Red in that episode. Secondly, at the beginning of episode 5, Elsa says in her narration they “crossed the main stem of the Brazos, just north of the Clear Fork.” The Clear Fork is that southernmost fork of the Brazos, which I highlighted in yellow below. Just north of that would be about where I added the red arrow to the Reddit map, which again, puts them going the wrong direction. The black dashed line that was originally marked on the map is not crossing the main stem of the Brazos, which is clearly contradictory to specific details provided by the show.