...they just added fake grain to the SFX, so they blend in to the natural grain of the live action footage.
Not only blend the sfx scenes then better into the 100% live action shots, the sfx scenes also look more sharp and natural due to this.
And that's how it is done!
Yup, exactly how the restoration team did it on The directors edition of The Motion Picture. They sampled the grain and had a grain texture that was applied to any new effects to make them match the original footage.
Why HTV did it backwards is somewhat baffling.