It appears the company says it's not meant for shouldering as that would put the slide too close to the face when it cycles. It's meant as another point of contact. And it is holsterable.
https://www.thefirearmblog.com/blog/2019/02/01/flux-brace/
https://www.fluxdefense.com/
https://www.thefirearmblog.com/blog/2019/02/01/flux-brace/
https://www.fluxdefense.com/