Yeah, it's unlikely that Powell suddenly got better at 29 years old after and 6 years spend in the NBA. Playing more in China and Puerto Rico could have helped him but I doubt enough to turn him into a legit NBA player.
Saying that, this thread is premature. Spurs have still 5 preseason games left before facing the deadline to make a choice on his contract. They can give some minutes to Powell in these 5 games because they don't really have other PFs that needs preseason minutes. After these games, it will be time for Spurs to decide whether or not to keep him. By that time, Powell could have faded away turning cutting him into an obvious mode. He could too still be damn impressive making his recent play being lees likely a fluke because the sample would be bigger. If Powell is still playing well, keeping him could be an easy choice because other invites could too have faded away.
Right now, I would keep Curry and cut Powell but it doesn't matter because Spurs don't have to make the choice now. Near the end of the training camp, it will be time to decide who to keep and who to cut.