10% fewer instructions but average instruction is longer, so code density is the same, they claim. This still leaves their ISA with the worst code density of any non-obsolete ISA.
I've only read one study [1] and its follow-up [2] with code-density benchmarks but according to them (one source), x86-64 is actually one of the denser contemporary ISA's ... provided that the compiler/programmer is smart enough to adapt to the ISA's quirks.
1. <https://www.researchgate.net/publication/224114307_Code_dens...>
2. <https://web.eece.maine.edu/~vweaver/papers/iccd09/ll_documen...>