Building Prismatic is way more than just about having a formal ML background. It's that along with large-scale systems skills and having a sixth sense for working with data (text especially) and knowing what simple ideas will work and what needs to be complicated.

So to answer your question yes, you need a formal ML background but you need a lot else. Luckily, you can pick up all these skills from online courses, real world building, and a lot of self study and improvement