Biking is more efficient than walking because of the mechanical advantage of pedaling, right? But it can’t possibly hold true for any incline can it? Like no matter what, you are always moving more weight than on foot. So then would the efficiency factor have everything to do with incline?

I mean you move more weight on flat surfaces, too. The problem with going up inclines is going to be you need to fight the wheels rolling backwards which doesn't happen on foot as much due to shape.

I did a quick search for it and found conflicting information, allthough going by what feels right I'd say this blog pointing out a critical slope (depending on different factors such as bike, terrain, and rider, allthough generalized for the article) where walking becomes more efficient.

I'm saying it feels right because if you ride up a very gentle slope, say 1-2%, you can still pretty much bomb up it with a bicycle at speeds that'd have you sprinting on foot without too much extra effort but on a steep enough slope all your pedaling is probably only going to make you not roll backwards, at best.

I know personally biking up steep hill usually feels harder than walking up it.

Just from personal experience, hills get much, much easier once you reduce speed. This is again unscientific, but something about sitting on a bicycle makes many people, myself included, measure effectiveness by speed and speed alone, disregarding things such as wind or inclines so we try to go up the hill about as fast as we'd go on flat ground and then it gets frustrating.

If you shift down and go a bit slower at the same energy output, it becomes a lot easier and depending on the incline, as mentioned above, you're still probably going to be faster than walking it with the same energy. Like 12kph isn't exactly hard to hit on a bicycle even uphill and it feels slow, but trying to hit that speed on foot uphill is going to be much harder.