Zork was originally a public-domain mainframe game called Dungeon developed at MIT. Its authors founded Infocom, split the game into 3 pieces, added more content, and released it for microcomputers as the 3 Zork games. The source code that's been floating around since the 80s is for the original Dungeon game. Between the early 80s and the early 90s, the source was translated from MDL to DEC FORTRAN to Unix f77 to C, so you can find a variety of copies of the source at different steps of that translation process. This is also why the C version doesn't look like idiomatic C code.

When Infocom shut down, one or more of the employees took home backups of the Infocom file server. Various partial releases have been leaked publicly from those backups, including tooling/language documentation and the ZIL source code for every Infocom game. The ZIL source code has been public since 2019. The notable thing that Microsoft is doing here is clearing up the rights to the 3 Zork games (but none of the rest of the Infocom titles).

I played it as dungeon on a dec vax, probably using a vt100.

Not PD, but free as a free ber and non-commercial.