She doesn't know how far back she goes in the Doctor's timeline. The fact he hadn't regenerated into the face she knew doesn't necessarily prove either way he hadn't met her yet. It's like if River met a version of the Eleventh Doctor with a hair style from an earlier point in his time line. She wouldn't assume he hadn't met her yet.
Plus, the Tenth Doctor could have met River between "Journey's End" and "The End of Time" from his POV (We know he spent at least 3 years in this period, a large portion of it avoiding death (Regeneration) after "The Waters of Mars")/between telling Amy that the Doctor's alive and Darillium from her POV (She can control her aging, so this could cover just about any amount of time.)