First, there's no requirement that signalling be costly. If there were, then "costly signalling" would be a redundancy. We engage in cheap signalling all the time.
True enough. Many are, though, and differential cost between accurate and inaccurate signals is important in such cases. Non-costly signals get subverted more easily. And, most of the time, non-costly signals are merely cheap, not free, or have a high cost at low probability when faked (for example, lying on your resume).
Second, a signal given to "gain the societal safe harbor protection" is still signalling. Indeed, this is a common motivation for signalling, displaying signs that tell people "I am one of you, I fit into your community and satisfy the conditions you expect of your in-group."
I think the fact that a behavior is partially about signaling is a very different claim than "rational astrology is the same as signaling". Not all behaviors in one category are in the other (there are non-astrology signals), and there are non-signaling reasons for engaging in the astrology.
More importantly, I think the two models of behavior are different as models, even if they predict similar results.
There are certainly non-astrology forms of signalling, but can you name any non-signalling benefits of "rational" astrology? It seems to me that this link is really just covering some examples of signalling.
The article can be found here. While it is not, for many of us, new ground, it is an excellent treatment, and it requires no rationalist background in order to be understood. The subject is the pernicious pull of doing the standard thing, regardless of whether or not the standard thing makes any sense, and it does us the service of giving that phenomenon a descriptive link we can share as well as an excellent name.
I hope to, after more discussion and thought, write a main post on the subject.