For a surname project you don't need to test the entire population of the world. You only have to test sufficient people with your surname to establish how many people with the surname are related. The standard practice is to test at least two people from each documented line to make sure that the two results match up and there has not been an NPE (non-paternity event). Some surnames are very rare and you would only have to test a small number of people to learn about the surname. Other surnames are very common and you would have test large numbers of people with the surname to get any insights. The number of people tested worldwide is therefore irrelevant. The important point is how many people with the surnames you are interested in have been tested and how many different lineages for the surname are represented. Some projects are very mature and have tested lots of people, others are just starting up and still have a long way to go.
You have exemplified why a DNA test for a surname such as Etchells is as relevant as a bucketshop heritage peddled by fraudsters.
When a surname group which is formed from a habitation name such as Etchells there are many thousands if not millions of primary sources to that name.
In other words there is not just one genetic lineage but thousands of genetic lineages.
It follows that to get any worthwhile data the DNA database will have to be large to enable to determine if one lineage is associated with other lineages for the same surname.
The most likely scenario is that large numbers of different unrelated lineages will develop from the one location and that those unrelated lineages will probably be contained in other surname groupings.
In a similar manner what those supporting DNA ignore is that if the total DNA database worldwide is only 1% of the world population all there theories of how the lineages are linked is only that a theory.
Until significant numbers are reached, meaning at least 50% and more accurately as close to 100% coverage is reached DNA testing is as accurate as sticking a needle into a parish register and choosing a name.
The reason being is until close to 100% testing is reached there can be no assurance that DNA sequences are unique.
It could be (though I doubt it is) duplicated millions of times in untested unrelated individuals.
Cheers
Guy