4
I cannot speak directly to how Jansson worked, but I have done hand work like this for posters in the 1980s and from the looks of the images in the search you shared, then the answer is these were "hand separated." However, rather than break up a single artwork using color filters into constituent parts, these would have been submitted as n- ...
3
The image you linked to is too small and low resolution to be able to tell you much.
I found a better version here. It looks like solid/spot colours. I can't really see the halftones even at that larger size, although they might be there.
This is a guess, but I suspect it's all hand drawn and the separations prepared by hand. She wouldn't have had to paint ...
Only top voted, non community-wiki answers of a minimum length are eligible