There was a huge difference between Inferno Body and Flame Body.
If with Inferno Body, Leon's body was still considered flesh and blood, then now, after having evolved into Flame Body, he could be considered truly immune to fire. Even without relying on his Fire Lord's Scepter, he could now swim freely in the lava ocean anytime he wanted and he wouldn't be hurt by the high temperature.
After all, the so-called fire immunity cast externally could only spare him from lower grade Fire element damage. Once an element flame went beyond a certain degree, he would still bear some damage caused by the flame. Previously, during the strengthening process of his Inferno Body, his body was easily damaged by the brutal modification performed by the Fire element energy. The strength of the Fire element energies had gone beyond the maximum endurance limit of his fire immunity.
