How is Bixby's assertion that Twain must learn the river better than he knows his own home an example of exaggeration?

In Twain's famous work, "Life on the Mississippi," Bixby's
statement, as he tries to teach Twain how to pilot the Mighty Mississippi River is
probably not as much of an exaggeration as it might appear at first.  He has been more
than a little annoyed at the lack of information Twain seems to have retained during
their time together; when Twain says that he thought Bixby was sharing all of this
knowledge, specifically the names of landmarks and geographical features along the
river, because Bixby was trying to be "entertaining," the tutor becomes very irritated,
but when his initial irritation wears off, Bixby says to
his apprentice: 

readability="10.854625550661">

'My boy, you
must get a little memorandum book, and every time I tell you a thing, put it down right
away. There's only one way to be a pilot, and that is to get this entire river by heart.
You have to know it just like A B
C.'

So, yes, this might be
construed as further exaggeration, or hyperbole, but perhaps not the greatest
exaggeration ever found in literature, as it was very important for Twain to know the
river if he was going to pilot it.