I would hazard a guess you have to use their proprietary material to inject into your mold.

No need to guess:

“This allows you to mold soft plastics (which you can buy from the company, also called Saltgator, or a hobby shop or fishing-lure supplier) and mold a piece up to 250mL (8.4 oz) in volume in about 15 minutes.”

From their FAQ:

Can I use materials other than your SoftGel?

Yes. While we’ll offer our own optimized SoftGel formula, SALTGATOR is not locked to proprietary materials. You can experiment with other low-temperature thermoplastics that match our safety and flow specs.