This is correct. The Russians cannot “clear the blocker” and then allow tanks to move through during the combat.
Combat is declared, and at that time, units can either make it to the combat or not. This should be an absolute rule.
If the territory was not occupied by anything but a control marker, you could blitz through it, then join your other forces and attack from 2 sides. Indeed, you could even retreat into the just blizted territory, provided at least one unit came from that direction.
Mechs cannot even do this; if you had 2 tanks and 5 mechs trying to join the battle by blitzing the control-marker only square, only 2 tanks and 2 mechs can proceed during the combat to join your other forces, because a Mech can only blitz in tandem with a tank.
In order to accomplish what you are thinking, requires coordination between different teams. One team kills the blocker, and then a second team attacks through the gap on its turn, before the enemy turn (this is known as can-opening).
Any single piece, on land or sea, blocks all land or sea combat movement in the way you contemplate. The only exception is air flying over it.
Despite all this, you could conquer all the contemplated territories and the NONCOM reinforcements into them. All the territories would have to be under your control during that phase, but that’s another way to partially accomplish what you are saying (not by bringing in reserves by blitzing but by backing up a victorious army during NCM)