The internet and the real world don't exist in separate vacuums. Internet communication and amplification of hate speech helped create an environment where hateful sermons were planned and popularized, and vice-versa.

Facebook pages and posts espousing these viewpoints (using sensationalized or completely fictional justifications) were then amplified by the algorithm. The amplification reached people that were not otherwise inclined to participate, but were convinced through information they received online.

You can't have one without the other. Influence is driven through both in-person physical events/marketing and online platforms. Categorizing the online influence as a "sideshow" completely misinterprets how important online communities have become in radicalization. "Reach" is a key social media metric for a reason.