How are these apes able to recognize that a certain toy requires motor skills and another toy requires nurturing?
One possible explanation is that the young girl apes would have watched the adults in their group and noticed that the females did the nurturing whilst the males didn't, so those young girl apes were simply copying what the adults were doing.
Human dolls aren't all that different from baby apes, less hair obviously but still built along the same basic structure: two arms, two legs, etc. so it's entirely possible that the young apes recognised the dolls as approximations of baby apes.
I remember there was a news story years ago about a kid who fell into a gorilla enclosure at a zoo and knocked himself out, one of the adult females came over, picked him up and basically took care of him (even chasing off some of the other gorillas when they got too close) until the zoo keepers were finally able to get her to put him down so they could get him to the hospital. So it goes to show that apes can recognise humans as being ape-like enough to trigger their nurturing instincts.
IIRC the "boys" toys in question were simple things like toy trucks and stuff like that, so just by picking them up and giving them a cursory once over they'd have noticed that the wheels go round or some other part moved about.
However it's only fair to mention that the study wasn't the world's most scientific, it was conducted at a working safari park so there was definitely opportunity for the results to have been skewed somehow due to all the humans passing by, plus like I mentioned earlier the young apes weren't separated from the adults before they could pick up behavioural patterns from them (though they were tested at quite a young, pre-juvenile age), so there's that as well.
But for all that it still makes sense and I wouldn't be at all surprised to see those results validated in the future. Female mammals are physiologically built towards nurturing after all, and it would be foolish to assume that the brain isn't subject to the same "rules of construction" as the rest of the body.