It's wrong because either sooner or later the players will realise it, or simply because the DM is deceiving the players. He knows that if they knew, they would feel railroaded.Railroading only occurs if the DM is removing player choice and forcing them down pathways, even if they are not aware of it through due to the illusion of choice being presented to them.
Lots of them make blanket statements, or at least did in the past such as "the villain must escape at this point" or "if the players do X find a way to prevent them so the adventure can happen". These adventures are often described at railroads in reviews. You can argue the reviews shouldn't use the term if you like, but they will keep doing it anyway.Looking at a published adventure and saying that "this is basically a railroad." is almost always going to be the wrong way to look it it, since next to none of them forbid the players from giving up and doing something else in the middle, or even saying no to doing it in the first place. If the PCs go on that adventure, it's because they chose to do it. That's linear, not railroad.
If you're definition doesn't account for common usage, then you're definition doesn't amount to much.