That is literally the same thing as saying it is not a literary masterpiece.
No it is not. It is saying something more than this.
If something is the opposite of something else, if is by definition not the thing it is the opposite of.
Yes. This bit is true, but you are making a logic error to assume ‘the opposite of something’ is the sum of all things ‘not being that something’
Examples.
If there is an exceptionally tall person, a medium height person and an exceptionally short person it makes sense to say the exceptionally short person and exceptionally tall person are opposities in terms of height. It is not logical to say the exceptionally tall person is the opposite to the exceptionally short person AND the medium height person.
The exceptionally short person is not exceptionally tall.
Similarly the medium height person is not exceptionally tall.
But only the exceptionally short person is the opposite in terms of height.
There is a vast quantity of literature. Some good, some inspiring, some mediocre, some terrible, some grammatically inventive etc etc.
A small section of this literature can be classified as literary masterpieces but it is illogical to say everything that is not classified as a literary masterpiece is the opposite of a literary masterpiece.
If you grade 100 poems from best to last and you pick up the best 5 ones and categorize them as exceptionally good you don’t automatically say the other 95 must be exceptionally bad. That is a logic error.
The opposite of ‘exceptionally good’ is ‘exceptionally bad’.
It is not everything that is ‘not exceptionally good’.