Sorry but I feel like this is way overthinking.

If mentioning gender at all and not specifically referencing your specific kind of non-binary identity feels like erasure, how is a blank toilet more inclusive than a quirky mythical sign?

The claim that centaurs are male and merpeople are female is weird too since that’s just your assumption. It’s like claiming cats are coded female and dogs are coded male.

What’s also being forgotten is that some non-binary people actually identify with mythological pronouns like Fae/faen for example. You might not be an alien, but some non-binary people do identify with aliens like in Ursula LeGuin’s portrayal of alien non-binary gender, there are non-binary people who strongly resonated with that.

To me the sign isn’t saying that non-binary people are mythical, but they’re inviting the viewer (whether cis, trans or non-binary) to see how gender categories are as made up as mythical creatures are.

Furthermore if you go to the Etsy shop for SassyCentaur there are so many different designs for this, not just merperson and centaur. Some more binary and gender coded than others. Some just show the Mandalorian and baby Yoda saying “this is the way". One just says “humans". One shows “all gender, with urinal". Or “all gender, with changing table".