Good question, ohr chodesh. I it allows me to clarify. I have no problem with a shul that says – to daven for the omud, you need, to be wearing a jacket, or a hat, or you can only use an ashkenazi haavoroh. If it is a minhag hamokom and identified as such, there’s an inyan of respecting minhogim. But what has been put forward here is different – as halocho, and articulated that it should be halocho for everyone.
Sam2 also had an excellent point.
As far as geneivas daas is concerned, the Gemara in Chullin 94a assurs misleading anyone in any way, Jew or Goy. Rambam also brings this issur in Hilchos Mechira and the S’A also does. So yes, it would be geneivah.