I tend to agree with many of the comments above, however I think this deserves a more positive take. I have the impression that runsy has taken note of criticisms and improved it. The latest version (4.9 being the one I've tried) is a good playable remix of MTG. It does exist to showcase runsy's mods, of which Petz absolutely is the star. It has a really pleasing visual aesthetic and it's clear that effort has been made to carefully select and integrate the mod choices. I get the impression this is aimed at being fun for younger players rather than to impress other developers. It's not a bad place to start.
The majority of players are not going to use developer tools such as git, so there is definitely room for packages like this that provide a playable version of MTG out of the box, particularly since MTG itself is no longer maintained.