If you're in line in the grocery store, not taking out your phone to fill those 30 seconds and just being there by yourself might be better for your creativity than opening an app and pretending that you have accomplished something in that time.
My opinion is that you’re wrong—30 seconds in the grocery line _is_ enough time to “accomplish something,” and Einstein / Toni Morrison / William Carlos Williams / etc were not merely “pretending” to be creative!
I agree with the GP that letting your mind wander in those situations is often better for creativity than trying to fill every minute with activity.