Assuming that you are referring to this design:
Then the answer is: there is a key difference.
Planetary transmission built by @Stanley Shi(2R) is a classic differential. It could easily add contributions to the output speed from both input motors but the output torque is limited by the weaker input (which in the above example is motor connected to the outer chain). So without some sort of locking mechanism that can prevent backdriving of the weaker input you end up underutilizing max power available from your motors. This thread has some additional discussion: Planetary transmission: can this really provide the torque of 2 motors? - Technical Discussion - VEX Forum and eventually refers to this video with the locking example:
Without locking, by the time you add up all the numbers, it will turn out you would have been better off just linking both motors in the direct drive configuration and relying on MC29’s to be the electrical equivalent of CVT to regulate your output power/velocity.
So the key here is to lock. You could do the locking via external actuation by pneumatics or servos, as many successful shifting transmission do.
However, the design revealed in the previous post uses the property of the worm gear to self lock (by friction) when output load (torque) is above certain level.
This is not a friction based transmission that I explained earlier (and had linked some lego videos) where max output torque is proportional to the friction.
Instead, it uses friction in a step function. The desired behavior is for the worm wheel to rotate freely below 1T load and be locked up somewhere below 2T. In my build it appears to be too well lubricated and step is clearly too high. Ideally, it should start locking much earlier, even before the steering motor reverses to cause some positive locking feedback.
Essentially, this transmission is related to Torsen differential (torque sensing) that has specially shaped gears that lock up when torque imbalance occurs between the wheels.
Sorry, if this was a little bit more of the explanation that you had expected, but it was a very good question, and I wanted to emphasize the key difference - which is locking, and that it is implemented without additional external actuator. The steering motor itself doubles both as the source of driving power and the “shifting” device at the same time.