"Less traveled by" could mean (1) less traveled at the time he made the decision or (2) less traveled at the (later) time when he's telling the story. I think #

"Less traveled by" could mean (1) less traveled at the time he made the decision or (2) less traveled at the (later) time when he's telling the story. I think #1 is the obvious meaning; your argument depends on taking it to mean #2. As for "Why would he sigh if he were not lying?", I hardly know what to say: what do sighing and lying have to do with one another? He expects to feel some regret, some sense of a missed opportunity, at never finding out what was down the path he didn't take. At least, that's my reading and it has the advantage that regret does sometimes make people sigh, whereas lying doesn't.