They're not counting total time spent with each toy. They're counting the length of each instance of playing with a toy.
So if the kid plays with toy A for 1 minute, then toy B for 1 minute, then toy A for 1 minute again, then toy B for 1 minute again, the number of interest for each toy is 1, not 2.
I would expect if the number of toys didn't have an impact on creativity or attention span, the following:
"Kids switch toys just as often, not matter how many toys they have" And I would expect that because children have limited attention spans - they want to switch after x time.
Yes, the cumulative play time per toy is obvious, but the individual sessions getting longer isn't very obvious.
The result implies, at the very least, that the kids plan out how much to play with each toy based on how many toys there are
What requires that they play with all the toys in an hour? A kid with 16 toys can play with each of them for 30 minutes at a time; a kid with 4 toys can play with each of them for 15 minutes at a time. "Length of session" != "total play time divided by number of toys"
Btw all the toys were new to them. So probably they just spend a few minutes with each to try it out. You're quicker if you only have to try 4 and pick one.
I must be missing something. Isn't this obvious? One hour of play time / 16 toys is less time per toy then one hour of play time / 4 toys.