The Great Xia Valley is certainly not a secret to those at the Mirror Immortal Camp.
Since the other side hasn't dared to come over until now, they must be aware of the Iron Refining Furnace. Xia Hong can now basically confirm that the incident with Wang He was orchestrated by the Deceitful Monster behind the Mirror Immortal Camp.
"Wang He didn't know he was being controlled at the time, just like the ones we just saw. But why was he afraid of the Solidified Fire Oil while these ones are not?"
Being afraid of the Iron Refining Furnace but not of the Solidified Fire Oil, that Xia Hong can understand.
Compared to the Iron Refining Furnace, Solidified Fire Oil is just its derivative and a one-time consumable. If the Deceitful Monster is a bit stronger, it's not surprising that it would find ways to avoid it, especially as the Iron Refining Furnace is only described as intimidating and lethal to Primary Monsters.