They are too imaginary not to be false.
What does this mean? It is so complicated.
My theory is "too imaginary to be real."
OP seems to have answered the question by commenting that he means too imaginary not to be false (where too means excessively, and to is just part of the infinitive verb form in to be). Probably the most common form of this construction is too good to be true (Google claims over 10 million instances there).
But it's worth noting that native speakers wouldn't normally describe characters in plays (or theories) as too imaginary. Far more likely is:-
They are too far-fetched [to be true, credible, believable, etc.]
far-fetched - very unlikely to be true, and difficult to believe.
Z is too X to be Y is a fairly standard format, meaning that the qualities X and Y at some point become exclusive, and item Z has passed that point; it has so much X that it can no longer be Y. You can rephrase this as So X that it can't be Y.
Conversely, too X NOT to be Y means that X and Y are linked traits; whenever you have enough X, you always have Y as well. You can rephrase this as So X that it MUST be Y.
Thus, if the sentence under question is "The characters in this play are too imaginary not to be false", then they are so imaginary that they must be false.