The theme is BEANS!!!
But more to the point, think of BEANS in the Unity Capsule Primitive sense. We want you to feel able to explore any of your ideas using the basic shapes because this jam is about TRYING and not about perfection. So many students believe they must produce such-and-such quality before it's worth sharing.
No!
Finish your work because this process of "getting it done" is a hard skill to learn, and low-stress game jam environments are where your beans can fly free!!!!
Hey, good question!
The actual answer is "neither"! You do not have to use primitives, you do not have to use basic shapes to explore ideas.
The idea about the capsules is just a single interpretation of the prompt. We wanted to share that interpretation because we wanted you guys to feel like you could release a "prototype-looking" game that utilized primitives. That way you didn't feel like you had to worry about artwork or modeling and could focus on the game design.
BUT!
You can do whatever you want as long as it fits the theme in some way...
