I also don't think you've seen a real whipmaster in action in closed space. Sure, trying to use it in a coffin sized space would be impossible, as would most weapons. But there is quite a bit of room indoors by maneuvering down the length of a hall and such.
Heck it isn't any more impossible to use a whip in an enclosed space as it is to use a katana or sword.
I won't pretend to have seen whipmasters in action in closed space - the closest was a pair of urumi masters in what I'm assuming was a display show (I couldn't read Sri Lankan) due to neither of them bleeding out at the end of the video.
However, I think you're being intentionally obtuse - a standard whip is easier to use than a monowhip which has a weight on the end in order to be effective. Could a monowhip slice through trees and small, low-hanging rafters? Maybe, but that's going to change the movement of the whip and throw off accuracy, even assuming that the weight doesn't get caught on something it's not supposed to (a glitch, perhaps). Nobody ever said or implied anything about trying to use something in a coffin-sized space, the mention was something indoors or when quarters are close enough that you've got friends and enemies all together. With longer weapons like a two-handed club, mace, katana, or longsword you've got to change stance and how you use it because you don't have unrestricted arcs of motion anymore. This is significantly less the case with smaller weapons like katar/punch daggers, stun batons, or the like, but what we're talking about is a whip where its advantage (and, if you think about it realistically, disadvantage) is its reach. You want to have as free motion as possible so you can send the weapon where you want it to go, whether you're using a monowhip, yari, or dagger. The latter two could easily enough be used in confined spaces or situations, the monowhip could not because of how it functions - motion (and motion control) is a large part of the damage a whip causes.