Their conclusions are confusing. The simplest explanation does not involve any bee psychology assumptions, but instead:

increased waggle dance from caffeine -> more energy exerted -> more trips to nectar to keep up energy levels

this assumes that caffeine has a stimulating effect the bees, but it still seems to be the simplest explanation

EDIT: now that I look at it, it's also possible that flowers containing caffeine have higher quality nectar under typical conditions, and so on....there are a lot of possibilities