Yes indeed sailplanes are a subset of gliders.
RE: @Federico's argument:
The quotation says: a sailplane (...) is a glider. And the same linked post about the tag usage proposes the merge...
Perhaps we could get by with just the glider tag and do away with sailplane.
... which was seen by many (relatively speaking since Meta doesn't get many visitors) and upvoted with no counter arguments to the idea of the merging/tag usage.
Thing is, if that distinction is to be kept without merging, then many of the glider questions are tagged incorrectly, as they are about sailplanes, much like how commercial-aviation is applied incorrectly to probably the majority of questions aimed at big airliners.
We could blame the site's beta stage, but what's done is done. I don't see a fix to that problem that keeps surfacing, and the proposed merge here would at least technically fix this specific issue, which is sailplane can't be applied to a hundred posts without way too much effort and drowning of the home page.
From the post you link
In particular, a glider is any aircraft designed primarily for unpowered flight. A sailplane, on the other hand, is a glider with conventional control surfaces.
I personally don't see how this can be interpreted to mean that glider and sailplane are synonyms.