Disney occasionally schedules nighttime spectaculars after park closing. We do not know why Disney does this, but it works fine: all attractions stop admitting new guests at the park closing time, but folks can remain in the park to wait for and enjoy the nighttime spectacular.
Also note that that Disney often changes its released park hours and show schedules. If you are looking at a date four months from now where fireworks are scheduled 30 minutes after the park closes, it's possible that park hours will change over the next 4 months to include the time of the fireworks. We always recommend monitoring the hours of the parks you plan to visit.