There's no difference between a "compiler" and a "source code transformer". It doesn't matter if you "transform" PHP to C++, or C++ to assembly, or (as Digital did in the '90s) x86 machine code to Alpha machine code. It's the same process, with the same problems, the same algorithms, the same solutions, and the same outcome. The only difference, for less direct routes, is lost optimization opportunities.
It's disappointing that this thing only doubles the speed of the code, rather than making it 100x faster, or 20x. There should be another 10x worth of easy pickings lying there on the table.