Usually this is more directly ascribed to Si than to Ni. I think the reason this was ascribed to Ni at all is the idea that all introverted perception is both more experiential than rational, but also more based on the subject. The difference, however, is Ni and all intuition really is focused on perception of associations that are stimulated by unconscious activity, whereas sensation perceives associations taken in directly in conscious sense apprehension (although I do believe on balance, sensation types do receive hunches and the like).
Thus, while a subset of these associations correspond to past memories, there are plenty which don't. Of course, some unfortunately treat Si as if it's almost totally past-experience based, but it's sensation, and that would make it untrue to the spirit of taking in sense data, which even in the introverted attitude depend on external stimulus.
All functions seek to produce new information if used well.
There is also, by the way, the socionics version that forms an association between Ni and Time, although I dislike the way it's used (I don't think time as in things like being on time or whatever is what this is about; more abstractly, Ni is the state of consciousness where psychic reality matters the most, over any form of external reality). The association is more or less akin to the idea that the intuition of a conscious moment carries the structure of a "moment of time" (just like the sensation of a state fundamentally carries with it the structure of space). In socionics, then, Ni is dynamic and Se is static.
If you want to synthesize this with the Jungian concept, basically the idea is that you can have either a focus on potential-intuition (which seeks the convey the absolute potential by projecting the unconscious onto the current moment in entirety, so that the potential is conceptualized time-independently, or you can be a little more mercurial about how you see potential -- and view it as changing with time, or indeed identified with time, because the flow of time is what gives the capacity, in the rawest sense, for things to have potential).
Ni types thus tend to be a little more comfortable with paradox and such things (with the attitude of "sure we could look at it a different way, but this way is the most pertinent at this moment, as it looks to me") where the criteria for this determination are rooted inside.
Anyway what you read, as I said, is probably based on the idea that introverted perceiving appeals to subjective factors (a subset of which are memories). And it's true memories probably are somewhat more related to the irrational factor than the rational one. Because, while they can consist of rational information, usually if the totality is remembered, the rational information is subsumed in a context in which it happened to be imbibed, a mental state.