If this is Athena and she's telling the truth, this is the final nail in Genocide's coffin for me I'm afriad. I really really hate the whole "the bad guy from the future is you / the bad guy is you from the future" trope. I utterly loathe it, and have done since episode 14 of Trial of a Time Lord. This sort of thing becomes a Damoclean millstone around a character's neck and in this specific case as a DC plotline feels like little more than a thematic tie-in to Darkest Night.
Not to mention that it's yet another iteration of the "big bad transforms Diana into its tool" trope that we saw scant months ago in Final Crisis. You know, pretty much the most contemptable abuse of the character in her long history.
As you point out, it makes an utter fucking mockery of Diana's horrified reaction to the loss of the lasso. The one thing that really made the character stand out as something other than the usual "dark golem double" of Diana, and it's been fatally compromised. Even Doomsday has an existence that doesn't intrinsically rely on the death of his nemesis to create him. Diana averts the war and Genocide never comes to be, essentially invalidating every act and emotional response.