This hasn't been true for at least 20 years. There's a classic paper showing evidence of this in 1993 (O'Keefe and Reece) in rats. And has been an active area of discussion both before and since. (Note that it's not to say that rate isn't important, but as a community no one has beleived that all the information would be encoded in rate for many years)
There's lots of good explainers here that link to relevant research: http://www.scholarpedia.org/article/Encyclopedia:Computation...