Comment by Supermancho

Dr Murphy's talk goes off the rails around https://youtu.be/a5140uJOUDE?t=1057 where he confuses the narrators' thought about a future act, with the perspective of a future narrator.

    I shall be telling this with a sigh
    Somewhere ages and ages hence:

Dr Murphy claims there is a contradiction, when none exists. The narrator traveled a path that seemed equal to another. It looked more attractive for reasons not fully explained. Then it could have turned out to be the road less traveled by. We don't know, because the last stanza is a thought experiment, not a realtime perspective.

Then Dr Murphy also gives a somewhat biased interpretation of the sigh that showed an inappropriate glibness. I think most people would be able to understand how the sigh implies there was a difficult journey, as is often the case with unworn paths. Summarizing a problematic journey on a road less traveled by, without being able to communicate the experience fully, can lead to a resignation of oversimplification. Casually, "I took the road less traveled by" rather than expanding on "it was grueling".

Aside from that, I see no compelling reason to believe "all the difference" would have an alternative interpretation of negative difference. The idiom has a common use as positive and it would be a sloppy clash of tone as an alternate use.