That would certainly work better as a literal translation.
The closest Polish equivalent for the word “truthful” that appears in the current translation would probably be “prawdomówny”. However, it is mostly used to refer to character traits (e.g. “Tom was the kind of person who always told the truth”) rather than specific situations (e.g. “Tom was telling the truth when he said that he loves her”).
So “Tom was truthful” can actually work pretty well as a translation for “Tom mówił prawdę”, but only if we assume that “being truthful” refers to a specific situation.