It would help if they gave an actual example instead of just saying “When I added more elements inside the wrapper element, it started working.” because just adding another sibling element to the code given is not going to do anything. I also do not understand the explanation they give and there are multiple people in the comments commenting on this part of the article as well.
The sticky position element needs a scroll container (nearest scrollport) if the wrapper is meant to be that it has to have a scroll overflow. If adding extra child elements (i.e. siblings) was to create an overflow and the wrapper had a fixed height and an overflow set to one of the values that will create a scroll container, then yes the extra siblings would create the needed overflow. But that is not what the author is suggesting.
Note that a sticky element “sticks” to its nearest ancestor that has a “scrolling mechanism” (created when overflow is hidden, scroll, auto, or overlay), even if that ancestor isn’t the nearest actually scrolling ancestor.