No, they do not have the highway back. There are several groups of Russian troops sitting on the highway from Zhitomyr and from Korosten have Russian troops sitting on them and the bridges have been blown. There might be places where the Ukrainians control the highway, but they have to control it ALL to get supplies in.
The Russian only have to sit in one place and control the air. They don’t have to move at all and watch the Ukrainians starve in the cities.
This is a dynamic situation that changes daily. Your info may be off, or mine may be off.
Today at a Black Sea port, Ukraine appears to have successfully damaged/destroyed? a ship transporting resupplies for the Russian troops. Even if offloaded, the Russians still have to transport it many miles to their troops, facing ambushes and attacks along the way. Anyone have any idea how many such ships Russia has available there?