Whoopsies.
May have is the key word here.
Defused dynamite. One lit cigarette. Explosive experts said it was possible.
There is a blasting cap in a stick of dynamite. Expose that and a cigarette can set it off. And if Cooper armed real dynamite in any way, it became more sensitive to ignition by heat/flame.
Over time, dynamite sweats out the nitroglycerin. So if it was real dynamite, since nothing was found on the tie, at the very least it was not likely very old.
Very polite to the stewardesses, except for the part where he lit up next to his explosives and maybe took them to hell with him. I doubt it was real. The science now appears time to support that, though doesn't prove it.