The actual FTE's and engineers here can absolutely feel free to correct me, but my feeling is that "ground testing" is an important and necessary part of product design and development... and "flight testing" is supposed to be flight testing in real-world use conditions.
Flying around several hours within gliding distance of the airport is certainly a part of initial flight test, even by large professional companies, but it is not "real world use" testing by any means.
The FAA minimum 40 hours of flight test is a minimum, and in the case of funky-wonky-chunky-monkey powertrains like this... 40 hours is probably not nearly enough. The FAA I believe now gives you a reduction in E-AB test hours if you use a certified engine (from 40 to 25??), but in fairness there should be a quid pro quo and a longer or more demanding type of testing for something like this.
I'm sorry to advocate for more or bigger rules, but I am saying this with the desire to protect the freedom/privilege/opportunity to build and fly experimentals in today's world.