Sounds to me like the rival repair shop was trying to "scare" you by suggestion a transmission fluid change wouldn't include draining the converter.
Having owned (2) Porsche 928's over the last 5 years, and having changed the transmission fluid & filters, I must add it is a simple procedure to line up the drain plug of the torque converter to drain the ATF from there as well (especially if the car is up on a lift in a service bay). The 928 transmission is a MB design, using MB filters, which looked to me to be the exact same filter on my 95 E320.
95' E 320 (96,000 mi)
88' Porsche 928 S4 (81,000mi)
96' Ford Contour (140,000 mi)
86' Jeep Grand Wagoneer (126,000 mi)