Though then Gowasu did say that Merged Zamasu had Goku Black and Zamasu's powers combined and then it was extended to no end.
Because that is the reality of the situation. Fused Zamasu was the most powerful fusion ever, able to wield Goku Black's supreme strenght and Future Zamasu's perfect immortality/invincibility and flawless regeneration capabilities. Fused Zamasu didn't just have a mixture of the two powers, but had endless power altogether.
The writers were pretty naive to create such a powerful and astonishing villain, hence why they add to come up with an unbelievable and silly way to defeat him, because there was no other way at all. And I'm so glad that Toriyama changed his original plan. Have Fused Zamasu be weaker than two Super Saiyan Blue fighters? What a stupid idea. The final villain of an arc shouldn't be that weak. And even what they did in the anime, that they said that Fused Zamasu was just half-immortal, was so dumb. When Goku and Vegeta fused in the Buu saga, it's not like Vegito was half-alive/half-deceased because one of the fusées was dead. No, he was 100% alive. It should have been the same in the anime, Fused Zamasu should have been 100% immortal. Such silly writing.
Which reminds me, it's so stupid that Supreme Kai Shin said in ep. 109 that Jiren is by far the most powerful enemy they have ever faced. So I am supposed to believe that Jiren could
easily stomp Infinite Zamasu, a being who merged with the very fabric of reality and took over the multiverse and invaded other timelines all in the matter of a few seconds. Yeah, right. It's just what sounds coolest now, screw every logic. Because why say 'Jiren might be just as powerful and dangerous as Zamasu was!' when you can just say 'Jiren is the strongest guy we've ever faced', thus sounding cool. Because who cares if by doing so you are denigrating Zamasu and the mastodontic feats he achieved, whatever sounuds cooler, right?