I agree with most of what Vic says, however real world experience is a bit different.
Actually this real world experience is only useful when discussing these particular airplanes.
The effect on flight characteristics that can be induced by the moment of inertia, of mass mounted an extended distance from the C.G is well know. Part of that knowledge is the understanding that until testing is done to prove what effect a change will make on any particular aircraft with well known handling characteristics, any prediction is a guess at best.
This is why the FAA requires extensive testing of modifications to previously certificated aircraft such as tip tanks, etc.
From the description, I also don't think this incident was the result of a fully developed spin, but I agree with Vic (as I have already posted previously)... Flight testing a new aircraft is serious business. If it has modifications making it different from the original prototype, there should be an even higher level of caution used.
BTW, the original RV-10 prototype was spin tested extensively. Since it was a 100 % new design, it was done carefully and systematically, cautiously moving from the fwd CG limit, to the aft CG limit. The airplane was fitted with a spin recovery chute system and it was flown by a hired test pilot who wore a personal parachute.
Because of the successful completion of this testing, any RV-10 builder can likely expect no surprises regarding the stall and spin characteristics of their own airplane. But if they have done any significant modifications, nothing can be assumed. Only testing of their own aircraft can prove whether it has the same characteristics as the original.