Just because there's no doorway, doesn't mean a secret passage doesn't exist. In fact, that's a very good way to ensure your secret passage remains hidden; only foolish people hide an entrance, when you can simply not make an entrance at all.
... commoner logic ...
The bigger problem is that someone involved in this adaptation's creation is unfamiliar with the original source material; that person's attempts to clarify story elements are actually going to warp and alter the plot.
Wait, he knew how to fight somewhat from the beginning, but was hindered by the physical limitations of his new body. Now he knows a masterful martial art, but he spent the majority of his time plagiarizing an Earth web novel. Even if he devoted time off-scene to personal training, he shouldn't...