Certainly the radicalization may happen offline. But in the cases at hand, it did not. There is a significant uptick in violence associated with online radicalization, and no corresponding uptick in violence associated with offline radicalization. You can say that the role of the internet is overblown, but the evidence is there.

Besides, it's a sort of weird position to take on an a) online forum b) of tech people that the internet does not have a significant role in facilitating communication that would not otherwise happen.