Based on this transcript of the episode, the line appears to be:
DATA: You [Dr. Crusher] said earlier that I am unique. If so, then I am alone in the universe. When I began investigating the exocomps, I realised I might be encountering a progenitor of myself. Suddenly the possibility exists that I was no longer alone.
The "earlier" Data refers to was this line by Crusher:
DATA: And what about me? I do not grow. I do not reproduce. I am considered to be alive.
CRUSHER: That's true, but you are unique.
There are two salient points here:
Data does not say that he is absolutely the only mechanical lifeform in existence, or even the only one built by Starfleet: Dr. Crusher does (although he doesn't correct her).
Second and more interesting is the word he does use to describe himself: "alone". This could mean one of two things: that he's totally unique (which is unlikely), or that he has no contact with other beings like him (which is true). A prisoner trapped on an island is not the only human in existence, but they are alone.
This may seem like a trivial distinction, but what Data wants is not an intellectual understanding that somewhere, in the infinite stars, there must be another Soong android analogue. What he wants, like anyone, is to be able to interact with another being that understands his life and his difficulties.
Indeed, much of the episode revolves around Data's own philosophical understanding of his life and his place in the lives of others. From the same conversation with Dr. Crusher, we have this exchange:
DATA: I am curious as to what transpired between the moment when I was nothing more than an assemblage of parts in Doctor Soong's laboratory, and the next moment, when I became alive. What was it that endowed me with life?
CRUSHER: I remember Wesley asking me a similar question when he was little, and I tried desperately to give him an answer, but everything I said sounded inadequate. Then I realised that scientists and philosophers had been grappling with that question for centuries without coming to any conclusion.
DATA: Are you saying the question cannot be answered?
CRUSHER: No, I think I'm saying that we struggle all our lives to answer it, but it's the struggle that's important. That's what helps us to define our place in the universe.
DATA: I believe I understand, Doctor.
From this, it seems likely that Data would not be comforted by the mere fact that another artificial lifeform exists somewhere. Dr. Crusher is sufficiently aware of that, and the obvious fact that Data is hardly going to forget relevant information, that she doesn't see the need to bring it up.
Incidentally, their conversations discount Lore because he was destroyed (by Data) in the fifth season, whereas "The Quality of Life" was in the sixth season. Also, it's hardly likely that Data would be comforted by comparisons between himself and Lore, given how he reacted at the time.