It could be a generalization, but it does sound like they're referring to Kid Boo. Then again, the original manga usually made a distinction between the different forms of Boo if it wasn't speaking generally, so this doesn't need to be Kid Boo at all.
[mention]Pyro[/mention] isn't Pure Boo merely Super Boo - South Kaioshin - Dai Kaioshin? I doubt it's refering merely to Pure Boo, unless he and Super Boo are actually different beings.
It is just same thing like when Kami was shocked that Trunks who defeated Freeza easily, was nothing compared to the Androids despite the fact that we know Freeza is a fodder at that point.
Also a lot of the fans think that Pure Boo is joke when Goku himself think that he really needs to train a lot when Boo returns aka as Uub later. That just shows how Pure Boo is not something to mess up with.
He knew about him at the End of Z, so he had to find out some way I guess.
As for the topic, it's obviously not talking about power since that would suggest Boo > Golden Freeza. When Dende says "formidable", he's likely purely factoring in both the lengths Goku needed to go to beat him (ie. relying on everyone's help with the Genki-Dama) and the potential threat Boo posed (would've gradually wiped out all life in the universe if not destroyed). Whilst Merged Zamasu did prove a greater threat in these departments, Dende didn't see the battle, so the statement still stands from his point of view.