He's not saying that. What he is saying is that are simple, non-production quality implementation in Scala is much more amenable to experimentation than a sophisticated, production-quality implementation in C++ that weighs in at 300x the LOC.
But a simple non-production quality implementation in C++ would also be amenable to experimentation and not have the bootstrapping issues as well as provide an easier starting point to incorporate more of the existing optimizations as desired.
That is true, but the author of Metascala wanted to write it in Scala. Other people are free to write a simple C++ implementation of the JVM themselves.
Probably not, because JVM users are much more likely to be more proficient in Scala/Java than in C++.