The statement is true because promises, when successfully used as strategic moves, require that you follow through on the promised action; you will not promise an arbitrarily large reward. You may threaten an arbitrarily large punishment, however, if you can be certain your threat will work so that you won$$t have to follow through on the threat. Severe threats are not a good idea in general because they may not be credible due to their size, they may damage your reputation $($future credibility$),$ and because of the ever$-$present risk of error $($misinterpretation of moves, and so on$).($Large threats are not automatically not credible, but it may be harder to establish their credibility.$)$