Actually they are very different. "Asian" in its racial use today refers to a hereditary designation, entailing persons from the east and southeastern regions of the asian continent - sometimes called "Oriental" as well. In other uses it can refer to persons from the continent of Asia, much as "American" can refer to both somebody from the American continents or in its use of Native American, somebody from an American Indian racial grouping.
The antiquated term "Asiatic" could conceivably have similar uses, though its base definition simply means somebody from Asia. It can be determined from the OED definition you gave that this was the usage employed, as East Asians are not particularly known for wearing turbans though central and southwest Asians are.
Urbandictionary.com constitutes a "primary source" for the contemporary use of the English language and is a particularly rich source for slang and racial slurs.
Not any more than Wikipedia or any other user-created "reference" tool. But go ahead - try it on some term papers and see how many F's you get.
As does the base definition of "Asian".
It can be determined from the OED definition you gave that this was the usage employed, as East Asians are not particularly known for wearing turbans though central and southwest Asians are.
So I am right. Thank you for acknowledging that.