You are right. We do not have definitive proof either way. But I have no issues reconciling my version of what happened, and my version fits with the key theme of the movie, which is that one doesn't have to be special to make an impact - the act is what matters. Stelline makes memories for a living. It makes more sense that she makes memories for many replicants (note I didn't say all, because it can be just some and achieve the same effect of robbing K of his special status) versus one unique memory implanted in some cop who may or may not discover its significance after being triggered by a date on a dead tree.
Also, K tells Joshi about the memory and Joshi thinks nothing of it, except "hey neat, wanna bone?" If it was so super secret and significant, wouldn't there be some safeguard around spilling it to anyone who asks? Especially for a cop who can be ordered to pour his head out at any time?
Freysa's comment lets K and the audience in on the revelation that K is not unique, no matter how much he wishes to be. That's it. But through his detective work, K becomes the one to discover the child's identity (again, theme, action is what matters, not what you were given) If he himself isn't special, by extension Stelline must be.
Funny this is probably one of the few things that seemed pretty clear to me. If you have another interpretation, I am game to hear it.