It's an educated guess.Is that a guess or confirmation?
I got a good look at Laura in that scene, both onscreen and in stills. I'll bet money it's just 2015-16 Sheryl in the wig and makeup. Audacious - I was stunned at the time amidst my tears over the scene - but by god if it didn't work for me. There is no chance it is anyone else AFAIC.
And that's definitely Sheryl's voice performing as Laura. She had FWWM Laura's broken voice, body language and delivery ("do I know you?") down again instantly. It was eerie, like she'd just been filming the movie the other day.