Good questions.
I noted on the GM Tuners site it says the transaxle is a performance built 4T60? so something is listed incorrectly, somewhere. It certainly does appear that is a TV cable and adjuster. I know on the L27 intake manifold I used on one of my engines, the small arm to operate that cable was missing from the throttle body but it was a bolt on part that could be added from an LN3 throttle body.
I think you are correct Series III engines are drive by wire but usually there is a cable operated throttle body that can be adapted. I know when I have seen articles about other engines on the dyno, they many times have a cable throttle body substituted. That very question appears in other forums from people adapting more modern 3800's to older models.
I am a neophyte on the ECM capability but I do know that there are no apparent controls in the PROM for transmission functions. Either the ad is wrong and it is a 4T60 hydraulic transaxle, or perhaps a piggyback controller was used. It isn't mentioned so that doesn't sound correct. Later models used a Powertrain Control Module, PCM, for all functions, rather than the two discrete units we have. Actually thinking about it, what controls the "e" transmission in a '91 Reatta? As far as I know the instrument panel and diagnostic functions are the same as the '90, so assuming the ECM is different or a PCM type is installed ???