the verbal form "aurait pu" is an instance of conditionnel passé. The (indicative) plus-que-parfait would have been "avait pu"
"Nul" used as pronoun is a more formal form of "personne" (in the meaning of "no one"). "Nul", just like "personne", already carries the negation , so there is no ...
No native speaker.
Nul n'aurait pu le dire
'aurait pu' is the third singular of the so-called conditionnel du passé. See
The sentence, as it stands, conveys the meaning of the sentences
No one could have said it. No one could have said that. No one could have said so.