I'm not a Republican. I like Mamdani,I would have voted for him if I were living in NYC. I don't actually think this tweet on Xitter is inherently racist.
Most people believe 9/11 was caused by radical Muslim jihadists hijacking planes and slamming them into buildings. Radical Muslim jihad still exists. It is not like there are zero radical Muslims out there who want to destroy Western society, subjugate women, kill all homosexuals, kill all atheists, and institute Sharia law. That is not actually a made up trope of the greedy right to suppress the lower classes, that is in fact reality. There are still women not allowed to learn math in some some countries.
The fear Gulianai has, which I do not think is irrational to consider, is that by voting for a liberal Muslim, it's going to lead to policies that make it easier for radical jihadists to take root in America, that somehow a liberal Muslim mayor may embolden radical Muslims with extremist views or make it easier for radicals to take root within more insular Muslim communities in NYC, even if it's just a result of a cultural shift in which people do not fear radical jihadists. (This is not my opinion of what will happen!) I do not believe this view is inherently racist or Islamaphobic. I don't think it's wrong to have conversations about this, as long as it's done while realizing that many NYC Muslims are incredibly nice good people who support women's education, LGBT+ rights, and liberal values.
It would also be understandable if someone viewed this xitter post as Islamaphobic and racist and disagreed.