What they're actually saying is "having stuff on our own web site is a pain in the ass, with versioning and all. Instead we puff our DMs Guild where we can replace versions and automatically every "buyer" gets the new copy. Plus, we can say certain stuff on DMs Guild is a legit +1 source for AL play."
Oh yeah, notifications.
Yeah, that's also a thing and likely super handy.
What they're not saying but probably thinking is "...and since DMs Guild is a storefront we automatically get customer tracking data such as what stuff Mystic-downloaders also purchased and how much cash they spent, giving us purchasing habits and clues to what Mystic-lovers really are prepared to open their wallets for".
Well, yeah. But they're likely already able to get that stuff by looking at who bought 3rd party psionics and mystics.
Which ties back to what they did say: "...so don't give away your stuff for free - we want to gauge purchasing power."
You say the above like it's a bad thing…
If there's a lot of mystic fans spending a lot of money, that incentivises WotC to put out a more mystic-focused product for that audience. If there's only a small mystic fanbase, they can prioritize other content, making everyone else happy.
What they conveniently left out was that the the dream of getting rich off of rpgs is preposterous. The only ones making any money out of this will be WotC.
Does anyone really think they'll get rich making RPGs?
As for no one making money of this but WotC, the modest sales of my artificer document disagree. It's a non-zero amount. A couple sales each week. Admittedly not much, but when it's a couple sales each week for a dozen products it adds up.
I'm not getting rich by any stretch of the imagination. But DMsGuild sales have allowed me to just throw money at whatever Print-on-Demand DMsGuild product I want. I can buy D&D stuff on a whim.
And the idea of freely sharing fan content - no registration, no paywalls - dies a litte more.
No one is requiring people on the DMsGuild to charge. There's no shortage of PWYW and free products.
What do you want anyway: freely shared fan content with no paywalls or people other than WotC making money?
But it's not like free fan content has gone away or remotely diminished… There's still an eff-tonne out there.