My best guess is it's a busy road so it's dangerous and not really meant for bicycle and walking. I've done 13 miles yesterday to get a comic book on a bike and this right here is the distance between my house and a friend's house I told I can come on bike because what I just did gave me a feeling I could do it but my ass hurts so not right now. But yeah I want to see how this would play out. Before I would walk but I took a bike to a comic book store because it would've closed if I walk and I ended up getting there in time. Took longer than expected something that should've been an hour probably took 2 or 3 hours. So yeah I can do half of that for sure.
It's just a legal safety net that they cannot know whether at the moment the route is cycling-safe. So while you can follow their route, real-world situations might differ and hence you need to think for yourself.
Over here they show the same warning for pedestrian and car routes. Which is sad, because it tells me that enough people blindly drive into shit based off of a routing app that they need to tell people to please not turn their 2 braincells off, as much as a difference that is going to make...