I did wonder about that (OSRM & Valhalla produce even weirder routes btw). I'm pretty sure I went this way after doing the Bliistock ridge (but it was 22 years ago). 

Once I reached a path I continued gently to Mettmen, where I opted for a second beer rather than getting the last cable car (so it would have been about 18:30 when I got there). This seems the obvious place to have crossed the stream.