on the other hand, it's also important to 'go with the flow' sometimes, specially around things you don't consider too important; whichever those may be (I've noticed that what is important is even affected by my mood).
In any case, it depends on what you value. If you don't care that someone is into astrology, then go with the flow. If you deeply believe in science and it's one of your core values that people close to you believe in that as well then a short and polite conversation is all I see happening. I'm of the latter kind. I'd have never made such a strong comment though as it is my experience that people aren't really open to change their minds on this (having done readings on a few dozen people as a kid). So why waste the time?
Flirting is an informal process for obtaining answers to questions about a potential partner in advance of any significant commitment or risk and without having to deal with matters of any particular importance; questions such as "how well do we communicate?", "do I enjoy spending time with this person?", "how do they perceive me?", "how do they describe me to others?" and "how do they respond when we disagree on something?"
Reacting to these situations with denigration and hostility is very unlikely to leave a good impression; though certainly it does yield valuable information to the recipient.
Why would I care if I left a good impression on a person using astrology to judge me? I wouldn't expect nor want things to go any further.
Meanwhile, it turns out astrology is pretty good at revealing people who mock and denigrate others for no good reason. Who'd have thought it? Maybe I'll start using that technique.
I think he drew an interesting correlation between racism and her astrological beliefs that she likely has never heard before, and perhaps will give her some much needed introspection. That is a far more meaningful outcome than whatever else their conversation about astrological signs could have had.