I'm not really sure how the DM's Guild release is supposed to work.
What it sounds like they are saying is,
"Hey all! So apparently nothing we do with the mystic is making everyone happy. Tell you what, why don't y'all just make your own versions of it to show us what you are thinking. Go ahead and put out a bunch of subclasses and powers and whatever. Then after the dust has settled, we'll take what we like from those things, remake the class with them, put it out for a playtest, and if it does well make a final version of the class. And yes, that means the stuff you made will mostly all be incompatible with the final rules."
Although they might be saying,
"Hey all! How about you make a bunch of subclasses and powers and whatever that you can add to the basic mystic we've already built. Then we'll add the ones we like best to a playtest, and release the final mystic from there. Most of the stuff you make is probably still going to be compatible."
The first one means they just need to see more about what people really want with the mystic, and are possibly going to do a major rewrite. The second one means they're confident with the class they've built, but want more options and are opening it up to fans rather than doing it themselves. Either one likely means that the playtest results were conflicted.
I'm not sure how I feel about either interpretation. Fans can make some good stuff. After all, we often like our own work better than the official rules. On the other hand, it just seems so driftless letting it float around like that and seeing what comes back. I also feel like I have no idea what to expect with a follow-up playtest now.