I might not have been clear (sorry!) but the DSL here refers to the integrated assembly syntax as opposed to LLVM/GCC's textual approaches. Both syntaxes are officially documented in the D Language Reference [1], which is arguably the specification for D the language, but since DMD came much earlier than other implementations, I think it's fair to consider the DSL to be designed alongside with D. Anyway LDC and GCD's inability to support the DSL strengthens my original point.
[1] https://dlang.org/spec/iasm.html