Nah, it's a dumb explanation that fails to recognize character growth. The simple explanation is that Cap wasn't worthy at that moment. He was trying to pull the hammer as part of a pissing contest and was secretly keeping the knowledge of Bucky from Tony. He wasn't worthy yet, but he could move it some because he had potential. In End Game, he calls for the hammer in his most desperate moment when all doubts and secrets have been revealed. In that moment he is worthy.
It's called a character arc
It sounds nice and clean to try to pin a Steve's potential character flaw on that one moment, but like Steve tells Tony, he wasn't sure it was Bucky at the time. It was heavily implied it was Bucky and Steve knew in his gut, but wasn't explicitly told so Steve wasn't lying to Tony, just withholding potential information. Steve being Steve though knew that if he was being honest with himself, that he was shown enough to make that connection and felt it was better to keep it to himself or not look further into it than to open that old wound for Tony, while potentially throwing his friend under the bus, who would have been under mind control at the time.
Civil War seemed to be one of the few times that Cap was really wrong about something (even if he did end up being right about his hunch of Bucky not being responsible for the bombing), but that moment wasn't Cap's arc. It was Cap's arc for that movie, sure, but Cap's overall arc was about finding his home, finding a way to move on from his past he left behind, finding his purpose beyond fighting, and finding a way to live for himself rather than just living for others. Fighting for Bucky was part of his journey to discover that doing what is right isn't just catching the bad guy or trying to prove the innocence of your friend, but Steve was trying to hold on to the only friend he had left from his past, and it was because he knew that person he used to know was a good guy. It's a decision that's made with some self-interest, but for a person who has always thought it's more important to serve and protect others, to do something for himself is a breakthrough.
I don't really think that's necessarily a "worthiness" trait or that Mjolnir is a lie detector to discover if you've ever cheated on a test. All Cap had to do was confess to withholding the truth that he wasn't even 100% sure about. Cap's character has never been in question, and I think that's the idea behind Cap always being worthy. His arc was never about becoming a better person like most people's arcs tend to be, it's about becoming a more whole person.