Well, I'm not saying it casts Ennis or the movie or that scene into some whole new light. As Barb says, everything about that scene is already plenty devastating. And I'm not saying this indicates that Ennis suddenly understands for the first time that he loves Jack -- I'm one of those who firmly believes that Ennis knew that all along, even if he didn't use that word. And I believe Jack knew it. And I believe Ennis expresses his love in many ways, most of them nonverbal.
But I also think that, given that Ennis was not able to say "I love you" while Jack was still alive, the fact that he's now finally saying it (assuming he actually is) to an empty shirt adds an extra touch of poignancy, tragedy and romanticism to an already heartbreaking scene. I know that what must be going through his mind is how much he longs to say it to Jack himself, but knows it's too late. (It's possible he even would have said it to Jack in November -- when Cassie used the word in the bus station, I believe Ennis suddenly realized how accurately it described his own feelings.) All of which make me feel just a tiny bit sadder for him than I already did.
And on top of that I'm amazed that -- after 13 (I think) viewings, long after I've been able to recite each line with the actors and anticipate every glance and facial expression and cough before it happens -- it's still possible to discover something new about the movie.
I would say my belief that he says it has gone from 0% to maybe 75%. (I think it's slightly easier to see, BTW, than it is to hear. But neither is very easy.)