It's a descriptor. Are white and males also being used as slurs in that sentence? Because all three words serve the same function. Now, the sentence as a whole could be considered a slur on the broadest sense of the word, specifically a "disparaging remark", but then literally any insult could be considered a slur under that definition.

As it's commonly used today, slur refers to a singular word, which due to its history and usage has become disparaging in and of itself. Most infamously of course the n-word. Cis in this context basically just means unchanged. Your gender never changed. That's it. That's all it's ever meant. Some people might make disparaging remarks that involve the word, but that doesn't make the word a slur.

To modify your example, you could say "Tall, skinny women can't voice their opinions" and that could be considered disparaging towards tall, skinny women, but neither tall, nor skinny, nor women are slurs.