I don't have a true Tessar to compare to, but the Xenar on my Rolleicord seems to behave and perform exactly the way one expects of a Tessar. I don't think I've ever heard anybody say there was a meaningful difference.

Agreed, though, that KEH messed up in advertising it as a Tessar---I mean, what if you'd been buying it as a Zeiss collector? For user purposes I think it's a no-harm-no-foul situation.