The docs tend to conflate DataNodes (a HDFS service) and Impala daemons. I think this stems from the original deployment practice of always colocating Impala daemons with HDFS datanodes so that HDFS data could always be read from a local DataNode.
I'm a bit pedantic so the conflation feels wrong to me regardless, but I think this will become increasingly confusing as alternative deployments without colocated HDFS DataNodes become more common (e.g. running against S3, running with a separate HDFS service).
E.g. picking an example at random:
This is wrong because the memory limit is for an Impala daemon, which is the process that does the actual sorting. So here I think it should be "Impala daemon" instead of "DataNode".