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...