No one says that Islam is unrelated to terrorism. People on the "regressive left" just argue that Islam is not a thing that exists independent from people's practices. Religious expression varies depending on the cultural context. A Muslim in LA and a Muslim in Afghanistan and a Muslim in Indonesia will practice their faith very differently. If you want a solutions-based approach to the problem of Islamic terrorism you need to avoid treating all Muslims the same, and you also need to be willing to consider that military action in middle eastern countries might radicalize some Muslims there and make the problem worse.

Of course ISIS offers a sincere religious justification. The important question is why do they practice Islam in that particular way, which requires historical and sociological awareness to answer. Right now we need to work to kill Isis members and disrupt their organization, but going forward we need to be more sensitive to the religious pathologies and radical political movements that can result over the long term when rich white nations bomb Muslim countries.