The Guardian of the Fiery Spring did not let the twin giants do as they please. It moved its body like a snake and tried to coil around them. Sirius did not let this happen as he extended the shadows in an attempt to rescue the twin giants.
Even if the twin giants are immune to the fire being released from the Guardian of the Fiery Spring's body, they are not immune to physical attacks from the golem. Adrian thought that the shadows would not be affected but he became shocked. The shadows got burned by the flames of the Guardian of the Fiery Springs.
This was the first time that Adrian has seen the shadow that Sirius controlled literally become extinguished. Sirius could make the shadows tangible if he wanted to, but they should not be burned into nothingness. The flame is definitely special compared to the fires that he has encountered before.
"No time! Chrono Lag! Chrono Shift!" Adrian shouted.