The rest of the world disagrees with that assertion.
But yeah, in some natural environments, with some kinds of bikes, you can beat walking. The Burning Man festival is a great example, but it is also a terrible place to live.
That's why in most cases, without smooth roads, bikes are not practical.
- Fat bikes are more efficient than snowshoeing or breaking trail on XC skis.
- CX bikes are more efficient than walking in mud.
- Fatter tire gravel bikes are more efficient than hiking through sand.
- A bike with a 51T cog and 28T ring will be more efficient than hiking up steep grades until balance at low speed becomes an issue.