A promise (waʿd) announces a benefit or reward, whereas a threat (waʿīd) warns of punishment. The issue is whether pardoning an offender subject to a warning entails falsifying a divine statement. The Sunni response examines the scope of the warning and whether pardon is an exception compatible with it.
This is not a license to say that Allah could make a false promise. The discussion carefully distinguishes breaking an assured promise from pardoning someone addressed by a general warning.
One must first establish whether a text categorically predicts punishment for every individual or warns a class of offenders.