Is there even a reputable source for this claim that Youtube is dominated by alt right personalities?
Depends on what you mean by dominated. Anyone can start up an independent channel. Can't really say if there's more non-left leaning commentary channels to progressive leftist leaning commentary channels. You also have the major network channels which swing progressive on YT, like MTV News and PBS digital.
The difference is that the independent "alt rightgamergatorermerghedyerallnazis" group dominates in views. I wen through the suggested leftist channels of that thread, and the biggest sub number there is hbomberguy with 176k. I thought maybe that douche moviebob was bigger, but he's 126k.
Here's the subs of the other side: Sargon 728.9k subs, Needledrop (Fantano's music channel) 1.2 million subs, Armoured Skeptic 416k subs, and sh0eonhead is like 500k subs.
The best I have on the left is Laci Greene with 1.5 million subs, but now that she says you have to make your arguments with facts and shouldn't deny science she is off the approved leftist shelf. Plus, she's dating a Gamergater. (420k subs)
The major networks don't get the traffic of the anti-Social Studies Warriors either. MTV News is 100k subs. The
actual alt-righters are even worse, with YTers like Millenial Woes at 30k and alt-right.com's YT channel at 29k.
Basically, all the ratings are in shitting on gaffers and dumb college kids.