I think the difference is in how extensively you broadcast it. I like gardening, but I don’t care if nobody knows that I do. I think this is evidence that I’m not doing it for signalling reasons. So in that sense it’s falsifiable.
We could think of a scenario where his neighbor shares information about his (the authors) garden. In this case, if the author would seek measures to delete these posts, it might be interpreted as “signaling that he does not care and even doesn’t want others to know”.
I agree with you that signaling always shows intention (ie wanting something). You can “want not to want something”, though (:
But I think that's far too broad a definition of "signaling." The original article says "Building social capital through signaling."
When you post about your gardening to see if there are any other gardeners you can connect with, I don't think it's necessarily to "build your social capital," yet you can prove that it isn't.