I notice that you ignored the part shortly after the "He whirled and danced around his Master's defenses, testing them to their limits" quote.
"I understand you now," he said, still trying to goad his former master into breaking his concentration. "You killed my father and kidnapped me from Kashyyyk, not just to be your apprentice, but to be a son to you. Was that how your father treated you?"
The intensity of Darth Vader's attacks redoubled. "I have no father."
The apprentice fell back under the rain of blows. The sizzling of fabric and a faint stink of burning skin told him that at least two of Darth Vader's misses had been horribly near, but he felt no pain. He, on the other hand, had definitely struck a nerve.
And moments later...
Reaching out with his left hand, he blasted his Master with Sith lightning. That broke the momentum of the furious onslaught, enabling him to stand and catch his breath.
It's after that when Galen manages to injure Vader as in the quote you provided. Interesting how it's only after he electrocutes Vader (Something Vader is known to be weak against) that he manages to gain the upper hand in the lightsaber battle. And this is after Vader manages to strike him at least two times (Indicated by the comment that he smelled burning skin, and heard the fabric of his clothing being burned), and forces the apprentice to use the Force to break away.
Perhaps he wasn't being godstomped by Vader, but Vader held the clear advantage in the beginning, and drove Galen to using the Force to break away from the saber battle when Galen angered him.