The PrimaryOnlyService framework code guarantees that a PrimaryOnlyService::Instance does all of its writes under the same replication term. This naturally leads to the monotonic writes property. The monotonic writes property is sufficient here because the DonorStateEnum::kDonatingInitialData update can only become majority-committed if the no-op oplog entry to generate the minFetchTimestamp had also become majority-committed. This makes waiting for w:majority here unnecessary. And waiting for w:majority can be problematic because the timestamp to be pinned can end up lagging behind the current oldest_timestamp value and therefore become ineligible for pinning.