This phrase makes an attempt to illustrate the limits of written communication.
The context of the first phrase is relegated to the simple event of a poet saying something. Either by design or by chance the statement is true.
The rest of the text notes that having simply uttered words that are true, doesn't by necessity mean that the poet saying them knows, or expected them to be true. Of course, even if the poet thought it was true, he may have known only a part of the truth, without understanding all of it.
However, if the poet did in fact know what he was saying was true with scholarly accuracy, he might not be able to describe it adequately.
Take the following statement as an example:
I have gold and its shiny!
By this simple statement we cannot determine how much the author knows about gold. He may have exhausted all of his knowledge on the subject in those six words. But the possibility exists that he also knows that the current exchange rate is roughly $1245/ounce, or that its elemental symbol is Au originating from the Latin word for gold, aurum, or that is melts at nearly 1948 degrees Fahrenheit or its atomic weight is roughly 197.
Even those few facts leave out a wealth of knowledge about malleability, ductility, resistance to corrosion, electrical conductivity, etc.
To often we judge statements as being more, or less then they were intended. It is impossible to truly know the intent of another mind.