It is a real, functioning emulator of the Macintosh Plus from 1986. See the webpage[1] of the James Friend, who ported the emulator to JavaScript, as well has his posts[2][3] explaining more detail behind the project.

[1] https://jamesfriend.com.au/pce-js/

[2] https://jamesfriend.com.au/why-port-emulators-browser

[3] https://jamesfriend.com.au/porting-pce-emulator-browser

(As an aside, the Readme file probably didn't work because no text editor was installed - the items in the HyperCard stacks collection typically only include the OS, HyperCard and the stack itself. If you try opening the stack it should work just fine.)