Many of the scabbards in the article are so curved that it would be impossible to pull a blade of the same shape out of them. I'm sure that's what arthem was referring to.
With the daggers that I have seen and handled, the blade is curved but does not run the full length of the scabbard. It is not unusual for the blade of a L shaped scabbard to be cresent shaped[1], meaning that it is quite easy to pull the blade out.
[1] https://www.rct.uk/collection/62815/jambiya-dagger-and-scabb...