Jiuchong didn't know that Su Yang's words were to deceive him.
There are no spiritual sources inside the Immortal Palace, but Su Yang has the World Melting Furnace.
The World Melting Furnace is also a Great World, with an abundance of spiritual sources inside. In the Heavenly Sect within it, there is a very large Spirit Liquid Lake.
Moreover, in the Rongshi Furnace Valley, nearly the entire area is filled with Spirit Liquid Lakes. Su Yang cannot estimate how much Spiritual Liquid is in these lakes, but the Kunpeng previously told him that the Spiritual Liquid in the Rongshi Furnace Valley is not simply equivalent to a single Great World's spiritual sources. The amount far exceeds what one Great World possesses, and it might be the accumulation of the spiritual sources of several Great Worlds.
Not to mention, the Spiritual Liquid in the Rongshi Furnace Valley is enough for everyone to use for a long time.
