Resolution:
maxbytes is used to limit the total size (in bytes) of all pending messages in a destination of a given queue, or for each durable/subscriber of a given topic.  It relates to the entire message size on disk, and not at all with the message footprint.

By "...limits the memory allocation..." we mean both main memory and file system storage, because each message that is stored on disk is also stored in memory or at least the message shell is.  So, by setting the maxbytes you are limiting the disk usage *and* memory usage, but the latter is just as a consequence of preventing the server to store more messages on disk because the destination reached its maxbytes.