-edit- yeah after further investigation, according to their "learn more about this data" button, the original source for that data is two publications, the one I linked, and the 2007 paper. The 2007 paper is primirly about comparing nuclear to fossil fuels, and mostly from the perspective of indirect effects from CO2. I'm skeptical of it's claims, as that's a bit too roundabout. The one I linked above is direct effects only (except that it includes cancer deaths from nuclear accidents), which I think goes too far in the other direction. I'd personally include deaths from particulate emissions from fossil fuel generation, but that has no bearing on the comparison between nuclear and wind/solar.
I don't know how they ended up with the plot they display given the sources that they have, but I trust the graphs in the 2016 paper more, which show much larger effects.
In any case, it's certainly not true that there are no credible sources backing up the claim. One can dispute it, I'm sure, but there are credible sources backing it up.
https://ourworldindata.org/grapher/death-rates-from-energy-p...
Even using sources from a decade ago, pro nuclear sources like world in data couldn't find any credible sources to back that claim.
And solar and wind were just getting started a decade ago.