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.
Biking is also pretty dependent on the individual and their setup. The elevation changes, distances, sustained speed, and terrain one individual and their equipment can handle can vary drastically with another person. Not to mention someone's tolerance for whatever the weather might be doing at the time while you're completely exposed to the elements on a bike or walking.
It's just them taking a "your results may vary" approach while covering their own ass.
Anecdotally, while driving in Colorado, I put in a destination that I was driving to in bike mode on accident. The destination was like 80 miles away from where I was and involved climbing and descending a mountain pass. Google Maps was very optimistic about how long it would take me to bike there...all without knowing my anything about my health, the kind of bike I have, if I would be able to bike at that elevation, etc. (being Google they probably knew)
I can barely even beat the estimated time from google maps while using an e-bike.