It is not soft starting.
Think of the current and torque at normal operating frequency. It is almost a straight line from no torque at synchronous speed to full torque at breakdown, about 2 to 3 times name plate. In that range the flux current (feeds the magnetic field of the stator) is nearly constant. The rest of the current is converted into torque and a tiny amount of losses.
The operating point depends on the load torque. Up to breakdown the current is determined by the load torque.
Now what I did not mention is the frequency and voltage. If we reduce the voltage the breakdown torque is reduced. The same relationships still exist but torque and thus current is limited.
At this point we can see that adjusting the voltage controls torque and current although they are not independent. This is what a soft start does.
Up until this point I haven’t mentioned frequency and speed yet. Synchronous speed is the frequency seen by the motor. With a soft starter ignoring cycloconverter mode we can’t change it, except that the percent of synchronous where breakdown occurs does go down a little wire voltage.
But in a VFD we can simply change the synchronous frequency. Now our whole torque/current line shifts. By changing the frequency we can adjust the torque/current and speed almost independently.
Normally we have multiple conflicting goals with motor control. We want to maintain a certain speed (or torque), accelerate (or decelerate) as quickly as possible, and limit torque or current, or indirectly thermal load. Often these goals conflict. For instance with a large fan we want to get it up to speed without burning up the motor. The motor can’t survive the heat load of starting for say 2 minutes. A soft start seems like a solution but all it does is make a bad problem worse because it also decreases torque and adds more starting time, exacerbating the problem. A drive can be set to limit current to say name plate. It can just let the motor run on the edge of stall for as long as it takes to get to speed while manipulating the frequency so that we get name plate torque at any speed.
That doesn’t mean soft starts are obsolete. VFDs do bad things to your costs in many cases, and have other negative issues such as reflected waves, shaft currents, shorter life, and greater cabinet heating that soft starters don’t have. So depending on the application soft starters are preferable to VFDs when they can do the job, just as basic starters are preferable to both.