You can just follow the links in this case. I don't know much about RationalWiki and am disinclined to "trust" it, but often when that site makes claims, the links they provide to primary sources are more than sufficient to support them. That's the case here.
All the information about them is cited and backed up by evidence, the fact the website is left wing (according to you) shouldn't get in the way of the factual information contained within.
At least half of "information" is just guilt by association, sometimes taken to ridiculous levels:
"Winegard clarifies his above comment and concern is specifically only countries below sub-replacement fertility (2.1 TFR), meaning (excluding a few countries in East Asia i.e. Japan, Singapore and Korea), the whole of Europe.[33] This a key obsession of white nationalists (such as Brenton Harrison Tarrant, the perpetrator of the Christchurch terrorist attacks, whose manifesto called for white Europeans to increase their fertility rates above 2.1)."
Winegard says make more babies, you know who said that too? Christchurch terrorist! Ridiculous.
Just associating is not enough. You need to explain why something is wrong, some things are obvious some are not. There is article about this on Rational wiki (https://rationalwiki.org/wiki/Racialism) where they present their opinion on Race realism.
By linking those two things (association with Racialism and debunking Racialism) you can "present a case" against Bo Winegard (although in imperfect way, the best proof would be to debunk his words, not associate him to the theory debunked in another article).
Let's at least be honest.