Yeah, sous vide relies on both temperature and time to kill bacteria, so you really need to make sure you have it at the final temperature for long enough. Normal cooking temperature guidelines kill bacteria immediately, so for chicken once it hits 165º you're good to go even if it does that for only a second or two. If you're cooking to 140º you need to make sure it hits that temperature and stays there for half an hour or so.
So you know it's safe to eat at 165º just by using a thermometer, but knowing that a piece of chicken is 140º doesn't tell you enough unless you know how long it was at that temp. Pretty much any recipe will account for this though and they build in a bunch of extra time for safety so in practice the risk is minimal.
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