That's well within what they were making in donations per month prior to using Patreon, though. They already had the support, they just added another interface.
But, to the point, it's not like you could accuse the Zach bros of "selling their own image as a brand". They just have this thing they are making, a healthy relationship with the community (but nothing histrionic like your average youtuber), and people just gives them money because they want them to continue making that thing.