I don't agree with this definition which may have arisen from the Forge (?). Constrained options is not necessarily an example of a DM railroading otherwise we'd all be railroading. Railroading is really more about a DM forcing an outcome of X even though the PCs have avoided X and chosen Y. To that end it is a degenerate form of play in a game where player choice supposedly matters. Constraining choices to the point where only the one solution will work is really more on par with "pixelbitching" as I see it, but it's still not railroading, even though I would say both are not good examples of how to play the game.
There's reasonable constraint, sure. I don't think that constraining options results in a railroad.
I think it's more about the presence of apparent paths, only to discover that really there is only one path that will be allowed. I think that this generally happens when the DM prefers a certain path for whatever reason ("but I mapped out the sewers and printed stat blocks for the monsters there" or similar).
I don't really see the distinction you're making except maybe that it's the outcome of the chosen path rather than there only being one path? Okay, sure....but I think being forced along one path only is pretty in line with where the term railroading came from.
In this case, sure maybe the PCs won't make it through the sewers, maybe they will....so it's not the outcome that's predetermined, just the fact that any other avenue of entry to the castle is unavailable, leaving only one means. It's the sewers for them, and nothing else. That's definitely pretty railroady in my book.
Sure, they could go into the sewers and then fight the monster they find there, or sneak past it, or somehow bribe it......there are still options, potentially. Unless the DM decides, no this thing can't be bribed in any way, and it has tremorsense, so you can't sneak past it, even if you're invisible, it doesn't speak common, so you can't reason with it....and so on.
These kinds of decisions on the DM's part push things further toward a railroad, I think. They can happen at different points. Any time the PCs have decisions to make, removing what should be possible paths to take, is what I'm talking about.