Just to be clear, you're saying "uname -s == Linux" => "/proc exists" right? Not trying to be pedantic; just having a TIL moment.
‘Mostly true’, as they say. If `uname -s` is not 'Linux', then it's quite unlikely there will be a Linux-style /proc with assorted non-process things in it. If it is Linux, it's probably there; technically as it's a mounted file system it might not be present, but I've never even seen an embedded Linux without it.