"You guys always had the Fusion dance, the Potara earrings, and other means of combining as last resorts, but those would be rendered useless now!!"

[Evil Vegeta]

Some people seem to believe this statement means Moro 7-3>Gogeta because Gogeta wouldn't be strong or fast enough to stop himself from being defused. The lack of a contradiction on Goku's end is another reason why they believe it's a clear as day statement.

Other people believe it was just a way of writing Fusion out of the story.

Then there are the others who believe Piccolo was smoking on something good. The same Piccolo did say Vegeta was never one to misread his opponent's power. He also seemed confident his explosion inside of the barrier would wipe Moro out altogether.

I think the 2nd option makes the most sense, personally. Looking at things as a whole, a Fusion between Goku and Vegeta in the Moro arc should be leagues beyond Moro 7-3. Old Moro regained Prime Moro's power and more through eating 7-3, but that's really about it. Putting him above Fusion seems like a huge overestimation of his power.

 

[Symbiote]

Instead of Gogeta/Vegito we got a Susano’o ripoff to save the day.

 

[Pocket-Gog~]

You may as well just write fusion out of the story flat out

 

[SSJ2]

I think Goku in Super is simply too stupid to realize that Gogeta or Vegetto would one shot Moro. Even Piccolo's IQ has been reduced to single digits, and he's still presented as being one of the most intelligent characters. I think Toyo simply wasn't thinking about the limitations of Spirit Fission when he wrote that general statement.