Calvinist doctrine says God instructs them to tell all men, including the reprobate, that if they believe in the death, burial, and ressurection of Jesus Christ and accept his sacrifice they will be saved. According to Calvinist doctrine those reprobated unto eternal damnation for His good pleasure (John Calvin-Institutes) are damned to eternal punishment and therefore are to be told a lie (according to their doctrine)
It’s not so much a lie as an exercise in futility. In the Calvinistic scheme, IF the reprobate repented and believed, they would be saved—but the catch is that since they are reprobate, they WON’T.