The conclusion is just that the anthology would allow the patron’s magazine to depend less on donations. Even if donations cover only half or less of operating expenses, an anthology might still decrease the need for donations.

We know most poems published in the patron’s magazine are similar to those in the BWR. That doesn’t imply the poets who publish in the BWR never get rejected by the patron’s magazine. Maybe only 10% of submissions are accepted; they’re still similar to what’s in the BWR.

We already know that the poets in the BWR give up the right to monetary compensation for the anthology. So we already know that the cost is nonexistent or negligible when it comes to paying the poets.

We’re told the BWR makes enough from the anthology to cover most operating expenses. That acknowledges that there may be operating expenses not covered by the anthology. Pointing out something the author already acknowledges doesn’t weaken the argument.

This points out a difference that could affect sales between the BWR’s anthology and an anthology made by the patron’s magazine. The BWR anthology contains stuff from famous poets that didn’t appear before. That’s not just an anthology of poems first printed in the magazine.