The Diamonds know of the true potential of fusion

The Diamonds know how powerful fusions between different gems actually are, but in order to keep themselves in power and make sure no random two or three gems get more powerful than they are, they purposely spread lies and misinformation about it being "disgusting" and all that.

That world-wrecking wave seen in Lapis' flashback looked to be some sort of combination attack from three Diamonds at least; it could have been a fusion attack. And even though Homeworld did do fusion experiments, it was just something which could have benefited the Diamonds had they been successful, or simply covered up and forgotten about if they were failures.