Another correct version, I think slightly better: "This couldn't be more authentic." The use of "couldn't" for "can't" here is subjunctive, not past tense. —This couldn't be more authentic if it were straight from Paris. —She couldn't be more rude if she tried. —That building couldn't be any uglier (no matter what they do to it).

This is as authentic as it gets - correct. This (is) / It can't be more authentic- now it's correct. If you don't understand something please ask.