It is "a true deliverance" in several senses, firstly, because both Gregor and his father want Gregor to be back in his room. Secondly, and perhaps most importantly, it is because Gregor finds himself in a very compromising situation as he is actually stuck and cannot go anywhere and thus is forced to have his wounded flank exposed to the anger and violence of his father. Note what the text tells us:
With one side of his body heaving up, he sprawled lopsided in the opening. His one flank was bruised raw, ugly splotches remained on the white door, and he was soon wedged in and unable to budge on his own. The tiny legs on his one side were dangling and trembling in midair and the tiny legs on his other side were painfully crushed against the floor.
Thus it is that Gregor is trapped and finds that he cannot move. His room to him represents his shelter and home, a place of safety, as his father does not enter there. To be trapped in such an exposed position is incredibly dangerous for him. Thus it is a "true deliverance" when his father shoves him strongly enough to put him back in his room.