Electro / disco is not the right dichotomy, as their music has always sat at an intersection between genres. Homework, Discovery, and Human After All are all heavily sample-based, and there are like 1 or 2 samples on all of Random Access Memories. That is the big difference.
I think it goes deeper than that. They practically defined the 2010s electro genre, with heavy use of aggressive synthesizers, drum machines, and vocoders for nearly all the vocals. RAM was cleaner, more understated, and very, very slick. You could be forgiven for thinking it was a completely different band. Of course all interpretation of musical genre is subjective. :)
Which is (in my opinion) "innovation". An innovation not everyone agreed with.
Instead of sampling from the 70s, they used artists that created music during the 70s.