Może być also means 'maybe' 'perhaps' and the translation as " It might not be the truth" should surely also be acceptable-może być?
Here, the negated word is "może". So it really has to mean "cannot". For your sentence, you'd have to negate "być". "To może nie być prawda".
I love how she says być without blushing...
English translation comes off as awkward, It would be more correct to say: "This cannot be true".