I don't have the context of the research you read, but I've seen my budgies try to socialize and play with all of the following in a similar fashion:

- other budgies

- my green cheek conure (who isn't sure what they're doing exactly, but still tries to humor them)

- cuttlebones

- my fingers and tip of my nose

- my wife

- various parts of their cage

- toys

Honestly, I think budgies are happy go lucky little birds that like to play with anything and everything