The text you've quoted is about how a random subset of the population is already immune to the issue of repeated queries, not that subsampling the population helps in any way. If you don't interrupt the quotation mid-sentence, it reads:
> Even so, some randomly-chosen users need not fear such attacks at all: with probability (1/2 f)^h, clients will generate a Permanent randomized response B with all 0s at the positions of set Bloom filter bits. Since these clients are not contributing any useful information to the collection process, targeting them individually by an attacker is counter-productive.
The whole of section 6.1 is not about how it only protects a single question, it is about how one ensures that the single-question protections generalize to larger surveys, concluding that
> This issue, however, can be mostly handled with careful collection design.