It’s not a leak, it just means it cant beat the opponent for the maximum it could by playing the exploitative counter strategy vs their tendencies. Instead it just plays gto which will win against any given non gto strategy, though not for as much as the exploitative counter strategy. Playing an exploitative strategy however leaves you open for exploitation and this goes back and forth until the players converge onto gto, assuming the players are (very) good.
That said, it did train by playing against itself (before the experiment against the humans began).