According to his manifesto the largest factor was a visit to France, not online activity.
The fact that he was a member of an online community that considered his views normal is extremely significant.
The "online radicalization" thing is complete and total bullshit: it's an excuse to ramp up the totalitarian surveillance state in a more obvious way than the "war on terror" -now it's a war on people who read things on the internet. Magic internet forums might make me into a terrorist ... or a hacker; better put me in prison!
He found an echo chamber that reinforced his beliefs, which happened to be filled with internet edgelords that egged him on.
Coming to “the truth” seeing the reality of the world at first hand is a much more stirring story than being radicalised by lies on the Internet. Plus it fits directly into all the other racist propaganda put out by white supremacists about the state of Europe and France in particular.