I would bet you $50 that DDG was handled/requested a blocklist of domains from a spooky US/UK govt associate and did not unilaterally create it themselves.
Also, the US/UK gov is not an entirely unreasonable starting point for a list. There are also lots of academic researchers, media, etc. Better to apply some standards than none at all. I'm sure whatever list they have is not perfect, but that doesn't mean they shouldn't have one.
Q: Who determines what is "Russian disinformation?"
A: Teams of "specialists" funded by the US/UK governments (Bellingcat, Atlantic Council, etc).