If your argument is that you're an expert, then you need to have the credentials you claim to have. Anyone can show the faults in a design, but he's explicitly doing novel calculations and analysis - ie not just reviewing someone else's work.

Now that being said, it looks like he never needed a professional license as he fell under an exemption, in which case I feel like they shot themselves in the foot. He's got previous experience doing the same thing he's examining - hydraulics and fluid flow analysis. Regardless of his status as "professional engineer", his previous experience sould qualify him to testify.