What DSL cusotmers have are asymmetric connections, not half-duplex. Even on ADSL, you can watch a HD video stream while uploading stuff. The only way the upload matters is if it prevents timely delivery of ACKs for the video being downloaded, and that's only a problem on the lowest speed tiers and when you don't have decent QoS.
Thanks for catching that error, as I typed out asynchronous I was saying to myself "this isn't right, the a is in the dang acronym" and then I was too lazy to double check.
