By launching to a lower altitude initially, then more of the propulsion effort is done by the satellites themselves, and less is done by the rocket. All other things being equal, this simply means put more fuel in the satellites, and less in the rocket. Total mass is the same.
But all other things are not equal. Specifically, the satellites have 'engines' which have over 10x the specific Impulse - ie. Thrust per kilogram of fuel used.
So it makes huge sense to launch the satellites to a super low orbit and use their far more efficient engines to boost themselves up.
With the downside being the satellites ion drive engines take a long time to do the job, and if they aren't strong enough to fight against the headwind of atmospheric drag during that time, then the mission will fail.
In this case, spacex just pushed a little far.