ash (and its successor dash found on other distros) is a POSIX-y shell rather than a sh clone, so it has all(? most?) of the POSIX feature set, whose syntax may indeed have been 'borrowed' from shells that came later than sh.
Not sure if there's a "parent" from which both ash and bash inherit the syntax or whether bash is the true source, but that doesn't really matter here.
All that said, it's worth checking to see if your system has a command on the PATH called [[. That has been one way that [[ support can be added to a system when the shell itself might not support it. Note that command names don't have to be alphanumeric like functions tend to be in a programming language (or other languages if you consider that the shell can be used for programming too), so [[ is perfectly valid!