Technically, the Cryomancers were hinted at having left Outworld back during the 3D Era weren't they? If you really wanted to stretch out the lore it could have been revealed that the cryomancers were from Edenia but got slaughtered when Shao Kahn realized they were too OP. Thus they still would have technically been from both realms since Edenia was merged with Outworld.
I mean... it's still a giant @$$pull out of nowhere and still is another bad retcon for Sindel's character as well as the cryomancer race.
1. Originally Cryomancers left Outworld after Shao Kahn went full genocide on them.
2. Skarlet's intro
Skarlet: "Your kin descended from Outworld"
Sub Zero: "How would you know that?"
Skarlet: "You have the blood of Cryomancers."
This intro alone neutralizes Sindel's bullshit story about Cryomancers being from Edenia.
Merging realms also doesn't convert your Blood. Jade and Kitana still retained their Edenian blood. (confirmed by Skarlet), so if Sub Zero has the blood of Outworld citizen, then no way he can be Edenian.
But... eh. Sindel is a walking contradiction at this point. At no point any of the devs, have used their brains while making Sindel.
Not only has she joined the group of people that claim to have killed Jerrod personally, but also her relationship with Kitana is a complete mess at this point. She claims to have lied about everything, but Geras who OBSERVES EVERYTHING says this to Kitana.
"Sindel cared for you, above all else."
And the same Sindel proceeds to knee her daughter in the face.
As for that excuse of "different timelines". Sure DLC chars like Terminator don't matter. They are not part of MK. But characters like Sindel are written as part of main timeline. Mirror matches are the only place where different timelines are being mentioned/used to explain the two identical fighters. But aside that everyone talks as if they are part of main timeline, just different time frames.
In short, they don't even care anymore about lore. They just write, for the sake of writing.