There absolutely is a platform sh interpreter. That is why when ksh came out, it was called ksh instead of sh, so that /bin/sh would still function as expected

There absolutely is a platform sh interpreter. That is why when ksh came out, it was called ksh instead of sh, so that /bin/sh would still function as expected (same with csh, bash, zsh, etc). And many of these will emulate /bin/sh behavior when called as such.