All interesting points. Jim is self-absorbed enough that I doubt he'd take much care to hide this truth from her. He's not shy about telling her how she needs to improve, so I'm sure he would have spoken this truth, as well. The description we have in the play is that she suffers from a slight limp, and no one else in the story sees it as Laura does. Obviously her brace and her limp were magnified in her high school mind, just as a tiny blemish grows to a volcano-sized eruption when we become too self-conscious. It's a slight limp, no more.
I think this is quite possible. My father was in a wheelchair the better part of my life, and because his personality was so boistrous and flamboyant, not many people noticed the chair. He was a double amputee, and also had several fingers removed. If he were in his truck or seated at a table, you would never know...he was self-conscious about his disability, but he lead an active life and didn't let it keep him from doing the things he wanted or needed to do.
I've always taken that statement as conversational "white lie" out of kindness. In fact, I think that is probably the major thing that he noticed about her. Not out of contempt, but out of pity. The last thing Laura wants is pity. But that is all she has ever felt toward herself, and that is all that she has expected out of others. We find what we seek.
Because of Jim's nature, I would have to assume that he is simply being polite when he says this. He perhaps did not notice it often because he was in a completely different crowd than Laura was. Jim is rather self-centered, although he appears genuinely interested in Laura at times. I find Jim difficult to trust.
Back in High School, Jim only noticed Laura because of her brace. They had a class together, Chorus and it was difficult for her to get to her seat, so she was the last one to sit down, after clumping up the stairs in the auditorium.
The only reason that he remembers her at all, calling her Blue Roses is because she was a sickly girl in High School.
Now he says that he barely notices the brace, because he is being polite.
"He treats Laura kindly, but during their conversation he reveals that he too is not entirely realistic, for he discounts the severity of Laura's problem and assures her that all she needs is more confidence."
Jim, like many teenagers, was focused on himself and his own popularity. In addition, Laura has enlarged her disability because she has so little self-esteem. Her mother doesn't even allow her to mention her leg so she has had no adult to help her deal with any embarrassment she feels. All this was magnified in her imagination because she has no other interests other than her glass collection. Had she had friends or other outlets, she probably wouldn't have been so self-conscious. She has turned a minor disability into a major problem and sees herself as a cripple while others do not even notice her limp or simply ignore it. So Jim's comment that he never noticed he limp ( which Laura enlarges to a "clump") is probably the truth. He had so many other things to think about that Laura and any problems she might have had simply are not at the forefront of his mind.
I would say that his response is genuine. Laura's discomfort with her brace magnified the effect in her mind. But, for Jim, it wasn't even noticeable. It's human nature for us to see our defects as huge, when in fact, they are quite insignificant to others. Unfortunately for Laura, her personality was formed around this defect, and she is unable to believe that others can see past it to see who she is. It keeps her from making friends, going to business school, having a future of any kind. The result of is that Amanda then obsesses about finding her a man, and Tom feels unable to pursue his own dreams.