I think they just don't care. Swift-lang looks to be completely different when you look at the examples[1]. Its syntax is more java-y then Apple's iteration.
Also Swift-lang is Apache 2.0 licensed, which I have a feeling Apple wouldn't use as a base for a new language.
There seems to be a tradition in corporate programming languages to reuse the name of a somewhat obscure existing language. Google called their language "Go" despite there being already a language called that as well.
Did they not know or do they just not care?