They need to explain how this change fits into the Forgotten Realms context.
All they really need is a single sentence. In the blurb about "how to use this race in other campaign worlds" for the Realms, put in a note along the lines of "In the Realms, shadar-kai are descended from humans who were transformed into shadar-kai by the Spellplague". That's literally all of the handwaving they need to do regarding it.
Of course this means reminding people that the Spellplague existed, which will likely also irritate people. So I couldn't fault them if they left the sentence out.