I could be wrong, but I vaguely remember reading in a Buick brochure long ago that the T stood for Touring. AKA Touring Type.
I've never read any literature that said the T stood for Turbo. The only official Buick literature I've ever read that says anything about Turbo is the letter announcing the availability of the 1987 WE-4 Turbo T package. Most people, even myself refer to the 87 model Turbo Regals as the Turbo T package. 84-86s as T-Types. Buick nixxed the T-Type name in 87 and replaced it with the T Package. You cold get a T-package Regal with black out trim but no turbo V6. Gotta watch carefully when shopping as I have recently seen some non Turbo V6 87 T Package cars for sale on Ebay and Autotrader.
The T package and T Type are two different things.
In 1979 Buick had a Rivera S Type, which is a rip-off of a Jaguar name. Then in 1981, it became a T Type and was extended to other models. In 1983, it replaced the Sport Coupe name on Turbo Regals.