Is it respectful to tell the truth? If no, then you’re right, he’s disrespectful, if yes, then he is respectful.
If he is in error, we should correct Spencer, but if he speaks the truth, well then that is the basis for any dialogue.
You can’t shut down debate on the basis being disrespectful or even uncharitable. We should strive for respect and charity, and try to dialogue, but dialogue requires two parties, and many Muslims are unable or unwillingly to speak about the darker aspects of their faith.