If it had been a low level adventure, where normal town rioters would be a deadly threat, and going to the other house a no-brainer…
… would it be a “railroad”?
I think not.
Its not the constraint of player choice thats the problem.
Its the constraint of choice clashing against game world sense.
I don't know if I agree.
"Well, I'm Altherian. I'll just go to the Altherian embassy; I'm not a Big Damn Hero, after all. I should probably get out of the way."
"A rioter blocks your path, and does some fancy karate moves in a threatening manner."
"Well, then I'll go to the Milandiri embassy, as I made friends with people in Milandir in my backstory; maybe I can meet one of them."
"A different rioter blocks your path, and does some fancy judo moves in a threatening manner."
"... Are we in a district with competing martial arts schools?"
"There is a break in the crowd of rioters in only one direction. That is the only direction where you don't see any rioters making threatening martial arts moves."
It's still a railroad, because the "ilusion of choice" ("We're on a street in a city that has a grid of streets and locations we can go.") is not real -- you must go in the direction the module specifies.
Now, as DM, you can totally run something else. "You get to the Altherian embassy. After you arrive, you see the Ambassador striding through the hallways, dispatching people to trouble spots. He approaches your party, 'Hey, we received word that <the necessary NPC> is in trouble. Could you go see if they need help?'" But that's not only not covered in the module, the DM is
specifically instructed not to let it happen, and provided with
ludicrous tools (an infinite number of maximum-level monks) to enforce it. And doing anything else means you're basically running a different adventure.
That's the Railroad. It's in playstyle. Of course, a module can't actually enforce a playstyle, but many authors try.