That RPM drop isn't from closing the throttle. It's from cutting spark, ignition cutout at first, and then from the inertia of the car overcoming the torque of the motor and DRAGGING it down to rev match. The throttle linkage controls the shift points and weather or not the transmission 'kicks down' a gear when you stab the throttle.
Modern cars that are full electronic don't use a mechanical linkage to signal the transmission to alter shift patterns now.
For an example of this, look at this vid: https://www.youtube.com/watch?...
Note how the kickdown lever is operated and adjusted, it's pushed by the throttle when you floor it. Not the other way around.
https://www.youtube.com/watch?... GM uses a different setup, again you can see how the setup is to signal the transmission as to what's going on. Alternate methods GM used were vacuum sensing, no physical linkage.
http://www.jalopyjournal.com/f... has a discussion on using C4 transmissions without the throttle linkage, note no worries about the transmission not somehow closing the throttle on shifts, 'cause it never did in the first place.
https://en.wikipedia.org/wiki/... has a paragraph on the governor in the transmission that is controlled via either a mechanical throttle linkage or vacuum, aka the part we've been discussing. Note the lack of any comment about it closing the throttle.
Ultimately though, if you want to see this in action, drag out your ye-olde automatic with a mechanical throttle linkage and get it on a lift so the drive wheels are off the ground and the car is stable and secure. Pop the air filter off so you can watch the throttle body butterfly(s) while a friend runs the car up through the gears. You won't see any movement during shifts.