Because not every GM is ready to improvise something entirely different than what they had prepared for the session. And the Tomb was just an example. If you are ready the improvise the journey back through the swamp, well then good for you. But the general example of a GM unable to improvise on the fly whatever players suddenly come up with after they have signaled, they are jumping here and then going another way still holds. Fifteen-year-old me would have probably been floored and struggled with a lot of that, for example.
Then if they genuinely are completely unprepared, they should answer honestly:
"I don't actually have anything prepared for doing this, and I'm not up to improvising it tonight, so this will need to be the end of the adventure proper for this week. I'm happy handling downtime activities, or any bookkeeping we've been neglecting, but actually
running the game will have to wait for next week."
I don't see that, in any way, as railroading. It is the GM being honest that their skills are not up to the task
right this very instant, but that with a little bit of prep work--presumably in the area of "NPCs that might appear, general information about the area, a couple landmarks plus ideas for more directions to explore, etc."--they'll be able to improvise for whatever gaps remain.
Otherwise, that would mean that if the GM ever calls a session early because they're feeling unwell, they're railroading their players. Or if the GM decides not to run a session because one player can't make it, and they don't want to proceed without that person, that's now railroading. Or if the GM decides to run a one-shot side story focusing on the three present players because two people can't make it for a couple weeks, that too would be railroading.
Any definition of "railroading" that would include these activities is, as far as I'm concerned, worse than useless. That is, it is
actively harmful to the TTRPG community.