They may not be some monolithic block, and this guy may be just a vocal minority, but there are significant numbers of migrants who migrate to countries, who do not appreciate the culture or societal norm of those countries. Are they going to stage a rebellion? Probably not. Could situations present themselves such as 9/11, the Paris attacks of 2015, the Madrid train bombings in 2004? Yes.
You live in the UK, right? I am sure you know that Heathrow is always on high alert due to the possibility of terrorist attacks.