Yes, I understand that what the poem says is that he'll say it made a big difference, rather than that it did. But there's a difference between not saying it's a real difference and saying it's not a real difference; it seems to me that the former is what Frost does and the latter is what ArisKatsaris says he does.

(My reading of the to-ing and fro-ing about whether the road is "less traveled by" is that the speaker's initial impression is that one path is somewhat untrodden, and he picks it on those grounds; on reflection he's not sure it's actually any less trodden than the other, but his intention was to take the less-travelled road; and later on he expects to adopt the shorthand of saying that he picked the less-travelled one. I don't see that any actual dishonesty is intended, though perhaps a certain lack of precision.)