I believe it is generally accepted definitionally that "mainstream" isn't radicalized (i.e. mainstream is defined as "not the radical fringe"). Can you think of an example of why that definitional assertion should be discarded?

I don't think it matches modern usage. I regularly see complaints about how suchandsuch politician with mainstream support has a "radical agenda" or is "radicalizing" their followers.

If we insist on the definitional assertion, it becomes a lot less obvious why we should care about the original claim. Is it even a problem for Youtube to promote "radical" content if "radical" just means "different than what mainstream news channels say"?

what spicylemon pointed out, and I find many 'mainstream' news shows to be radical and not fringe.

Sometimes it's little slants added by hosts around facts.. sometimes it's hosts adding in little details to what interviewees have said - once you start to see it, it becomes sadly obvious that it's radical bias, and yet looking at the polls or seeing other people's fbook feeds, and such - it's not fringe.

Sometimes it's fact being left out, sometimes it's only reported what can be shown as bad.. or good - and avoiding showing other details. I must say 'mainstream' news both right and left has been radical in their support or resistance of certain narratives.

True or not, this isn't what they were referring to.