The interesting aspect of the word "credible" in the preceding paragraph is that taken as a whole (disregarding the irreversibility of reaching sub-games) strategies exist which are superior to subgame perfect strategies, but which are not credible in the sense that a threat to carry them out will harm the player making the threat and prevent that combination of strategies.
The intriguing aspect of the word "credible" is that, when considering all strategies (excluding the finality of reaching sub-games), there exist superior strategies than those based on perfect play but are not credible because threatening their implementation would harm the player making the threat and prevent them from using that combination of strategies.