I don't think they mean the literal information encoded in the RNA. The fact the RNA exists is, itself, a signal. It shows that there was previously a signal that caused the RNA to be transcribed. And then once you start including concentrations and degradation times for all the various things involved in the system...

Well there's lots of information there which can be used to infer aspects of previous states. I assume it's more of this aspect that is being referenced, rather than what is encoded in the RNA.