Thinset on painted wall?

[Ozzlvlosis]

I bought some stacked stone for a backsplash installation. From the manufacturer it's recommended I use a latex modified thinset. However my kitchen is finished and painted and they also said I should only adhere to backerboard. I've seen on multiple sites that in this situation people rough up the wall by sanding so it adheres better. I want to do it the right way but I dont think I can get a refund on this. At the same time I can't have stone falling off my wall in a year.. Can anyone shed light on this? Thanks for reading.

This is the stone

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