I think the question here is whether Email Inline works with Sierra or not? Not if it works with later OS’s.
I have three machines that I use regular, two on HS the other Mojave, Inliner working fine on all. I’ve nothing left on Sierra so can’t check that.
I’d suggest though, given the age of Sierra now, it’s very likely you will hit issues with it, so not really sure it’s as simple as punting this one back to the dev: At some point, devs have to stop putting time into making their stuff work on old OS’s, especially one that is something like 3/4 systems old.
Coming at this from a different angle… Is there a reason you are still at Sierra, not at least High Sierra?