Actually he didn't win, the court simply said USPS has to demonstrate more than a
de minimis burden to avoid their otherwise mandated obligations to provide reasonable religious accommodations. The case was sent back to the lower court to determine the individuals case.
In other words USPS will have to provide more proof of harm in order to deny the worker Sundays off, but Mr. Groff hasn't won yet.
In a unanimous decision, the top court reversed decades of precedent in determining that employers like USPS have to demonstrate more than a de minimis burden to avoid their otherwise mandated obligations to provide reasonable religious accommodations. The justices sent the case back to a lower court to determine whether, given the specifics of the case, the Postal Service could come up with other means to keep a letter carrier on the payroll without requiring him to work on Sundays.
https://www.govexec.com/workforce/2...ps-bid-require-employees-work-sundays/388095/