There are no published study showing such frequencies like 50% of T in east Saudi Arabia. This is only known throught private studies, some high frequencies in a few Arab tribes from Saudi Arabia. Mostly of T in east Arabia could be of Persian origin.
The 50% should be out of the total Arab members in the T project.
Today is known that Haplogroup T originated around 40.000-45.000ybp accordint to YFULL findings:
http://www.yfull.com/tree/T/ probably around the Black sea (Anatolia or futher north). The most basally splitted branches are found in Germania, Southeast Caucasus, Flandres and Bhutan. This match the most basally splitted branch of the L haplogroup that is found only in Europe.