It's an ancient custom dating back to the days of knights, and generally courteous when visiting some one new or entering any professional establishment.
I don't read any conflicts here, in fact it seems the blurb you shared is speaking to normal food particle size that passes though, while the one I shared talks about maximum foreign object size that can pass.
I never threw the quarter i swallowed at 6 years old back up, doc said it likely passed. Thats hella bigger tam 1-2mm and there's no quarters showing up on imaging... so how exactly does that work?
Not saying I don't believe this its just that reconciling this statement with real world experience isn't adding up.
And now I'm picturing the 'Little Book of Calm' getting absorbed and Bill Bailey running around looking like Jesus and quoting it. I never walked around like moose jesus so I guess I didn't absorb it.
https://youtu.be/1xOOFCltZuc?feature=shared