The Ars article being discussed is actually where I read that. However, upon attempting to find the original case it looks like SCOTUS hasn't yet heard such. From this Lawfare post, (https://www.lawfareblog.com/fifth-amendment-decryption-and-b...), SCOTUS Fisher v. United States is relevant but doesn't actually involve passwords. Also from that page, the Eleventh Circuit seems to think that government knowledge of incriminating device contents permits forced decryption while the Third Circuit argues that merely knowing you possess the password is sufficient.
In both cases the precedent is that you can be forced to turn over a password; it's only the details of the prerequisite government knowledge that are under debate. As far as I can tell, the current PA case goes against that.
Referenced PA Supreme Court case: https://law.justia.com/cases/pennsylvania/supreme-court/2019...
Related PA Superior Court case that was appealed: https://law.justia.com/cases/pennsylvania/superior-court/201...