The short version: the first phase was intended to keep Ukrainian troops from re-inforcing the Ukrainian forces in Donbass. Having destroyed the means (military infrastructure) of re-inforcing the the Ukrainian forces in Donbass, the Russian troops can back of Kiev and send more troops to Donbass.

To which I answer: Maybe? Ritter is knowledgable, but he has been wrong on this war several times.

There is first off the question if this was plan A, or plan B. But that is a hard one because military plans tends to have back-up plans and there is no way to be sure until archives are opened what was the plan, what was a feint and what was improvision. And it is perhaps not even an important question right now.

Looking instead at the statements on the situation on the ground today. Has the Ukrainian army been unable to re-inforce the eastern front? The unability to lift the siege of Mariupol points in that direction, yeah. Will they continue to be? Maybe. If they continue to be unable to re-inforce the eastern front what will happen? Most likely Russia will win there, with the break out republics claiming their entire oblasts as territory. With their strongest force defeated Ukraine will be in a military bad position. But they already are, so it doesn't change the political logic of the situation.

In light of this, I hope that the partial retreat signals a diplomatic breakthrough, so that peace can be given another chance.