To understand it, let us take a simile.

Suppose yourself walking down the street with a man who continues to sprinkle the crowd out of a flask of vitriol.

You would be much diverted with the grimaces and contortions of his victims; and at the same time you would fear to leave his arm until his bottle was empty, knowing that, when once among the crowd, you would run a good chance yourself of baptism with his biting liquor.

Now my companion's vitriol was inexhaustible.
It was perhaps the consciousness of this, the knowledge that I was being anointed already out of the vials of his wrath, that made me fall to criticising the critic, whenever we had parted.
After all, I thought, our satirist has just gone far enough into his neighbours to find that the outside is false, without caring to go farther and discover what is really true.

He is content to find that things are not what they seem, and broadly generalises from it that they do not exist at all.