It's basically the same thing they did with Peacock (when they had the one-off local game not the national one) and what Apple TV did the first year (where you just had to have a device or get the app, not subscribe).
Teams get revenue-sharing from all national TV deals so this isn't really any different than having your game tapped for Sunday Night Baseball. And there are more people with Roku or Roku devices than there are ESPN subscribers. They aren't active of course, but the same is true of ESPN households (which is why people started complaining about the bundle in the first place). Peacock and Apple TV are both considerably smaller though those are all people who have already successfully established a log-in and subscription.
Of course there won't be good ratings, and there aren't for the others either.
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