I like the words used in both panels, "Psychic" and "mental powers."
Both could be synonymous with "Haki."
The first time, he doubted himself having such power back at Fishmen Island. That was probably the first hint / lie to his haki awakening later in dressrosa.
The recent one with him saying his mental powers did free him, could be a reminder by Oda that Usopp is on the verge of doing something of great importance again with his haki, as he's more aware of it now. (Some even say it could be his conqueror's haki awakening, soon).
Because freeing yourself from an opponent's grip / devil fruits power as law and others in the story had demonstrated to us resulted from their haki.
In the same way, Law stated that his powers wouldn't work on Kaido / Big Mom because of their strong haki.
This has happened several times in the story before, so these could be classic Usopp lies that will turn into reality, and have yet to happen. It could also be that Usopp will free other people again as he set the Tontatta tribe free.