My guess is that Singer knows he’d alienate these billionaires if he came off as too socialist. Given his pessimism about systemic political change, he probably sees the choice as between:

(A) Billionaires donate large sums of money to aid organizations to help worst off + No systemic political change

Vs

(B) Billionaires don’t donate their money + No systemic political change.

Perhaps his pessimism is unwarranted and he should reject both (A) and (B) as the only options. But until he’s convinced of another option, (A) is obviously the better choice, and it’s the choice that Singer is making, by not being too critical of billionaires.