English Translation
He wasn’t lying, he was speaking sincerely.
deveras
a. ǁ de a ~. loc. adv. Mx, Ho, ES, Ni, CR, Ec:S, Pe, Bo. De veras, con verdad. pop. (de deveras).
b. ǁ de ~. Mx, RD. de a deveras. pop.
(deveras | Diccionario de americanismos | ASALE)
I don’ now whether “ES” above refers to Spain, but then it wouldn’t be an americanism, would it?