What does "the choice must be consistent for all consumers" mean?
The important part is:
Otherwise, the exporter may provide either a read-only or writable buffer, but the choice must be consistent
The choice..this choice, because the first case is no choice...must be consistent you make for every consumer, not the thing.
If the field is set, you must provide a writable buffer. There is no choice, because the technical inability to do so for whatever reason is no choice.If it is not set, it can still be writable, but then it must be writable for all who don't have the flag set.