Sun Wukong is not a true immortal during the journey
The Daoist practices that Sun Wukong uses to achieve eternal life were historically thought to result in an immortal spirit that left the body. However, his practice results in an adamantine physical body.
https://journeytothewestresearch.com/2017/10/21/the-connection-between-monkeys-spiritual-training-and-historical-daoist-internal-alchemy/
This body is capable of carrying two celestial mountains while running at great speed.
https://journeytothewestresearch.com/2018/08/05/1521/
But he is still vulnerable to damage if the force is sufficiently strong enough. For example, the above episode sees Monkey pinned by Mt. Tai (the heaviest thing imaginable in Chinese culture), causing blood to shoot out of his orifices.
Another example of damage happens when he is engulfed in the supernaturally hot flames of Red Boy’s Samadhi fire, causing him to seek relief in a nearby river. However, going from hot to cold causes him to pass out, leading to Zhu Bajie to perform emergency resuscitation.
This may support one scholar’s theory that Sun is not a true transcendent—meaning he's just long-lived and really hard to kill—until he becomes a Buddha at the end of the novel. This is based on the Buddho-Daoist philosophy of Zhang Boduan (張伯端, 987-1082), who believed one must achieve both the Daoist elixir and the Buddha mind to escape the rounds of rebirth and enjoy true immortality.
https://journeytothewestresearch.com/2019/12/19/archive-12-huineng-subhuti-and-monkeys-religion-in-xiyou-ji/
As I've mentioned before, the Jade Emperor and the Buddha both refer to Monkey as a "bogus immortal" (妖仙), suggesting that they know he hasn't yet achieved true transcendence. There are other bogus immortals in the novel. My friend and I have created the designations Greater Bogus Immortal (大妖仙) and Lesser Bogus Immortal (小妖仙) to differentiate between powerhouses like Sun Wukong and riffraff like the True Immortal Compliant (如意真仙) from the pregnancy episode.
https://journeytothewestresearch.com/2020/12/22/origin-of-the-pregnancy-episode-in-chapter-53/
