How does my pre-cooked Costco chicken breast not expire for another 5 weeks?
Keep in mind that the best before date is only valid until the package is opened.  Once it is opened, you must treat the chicken strips as if they were fresh and consume them within a few days.
Most likely the chicken is using 'modified atmosphere packaging' (MAP).  It's a very common method used to extend shelf life of meats and vegetables but since it doesn't look a whole lot different than regular packaging, many consumers aren't aware of it.  Wikipedia has explained the process fairly well.  https://en.wikipedia.org/wiki/Modified_atmosphere