Huascar knew perfectly there was going to be an eclipse, but he hid it to save Tintin
Tintin giving up his medallion to save Zorrino convinced him definitely that the Incas were wrong on trying to kill Tintin, Haddock, Calculus and Snowy, so as the Great Priest he hid the fact that an eclipse was nearby, exactly on the chosen execution date. He saw through Tintin's desperate plan and played along with it. That's why we don't see him along the panicked crowd, or trying to appease it: while all they knew perfectly that it was a simple eclipse, they thought that it was actually Tintin who caused it (modernity had made those 20th century Incas lose familiarity with their actual customs). As a wise man, Huascar expected this and let it happen.Over time, the Great Inca knew about Huascar's ruse, but because he felt that his newfound friendship with Tintin and Haddock was genuine, he let them be, knowing he could have murdered innocent people because of foolish pride.
