As more data is indexed the system "learns" what makes a relevant search based upon the words in each document. Though I agree the definition of learning in a machine learning context is kind of grey.
It doesn't learn, it fully supervised and doesn't have feedback loop.
I don't say it's not interesting, it's step forward compared to other articles in my collection [1][2][3] about the same subject allows to dive easily into the metrics and heuristics used in IR.
I don't know if the definition is really that grey.
Tom Mitchell's popular definition (from his book "Machine Learning"), which is is even quoted in the opening of the Wikipedia article on Machine Learning, comes to mind:
"A computer program is said to learn from experience E with respect to some class of tasks T and performance measure P, if its performance at tasks in T, as measured by P, improves with experience E" [1]
[1]: Mitchell, T. (1997). Machine Learning, McGraw Hill. ISBN 0-07-042807-7, p.2.
