In thermal injuries the stiffening has a different mechanism and is unrelated to rigor mortis as explained below.
If the temperature that the body was exposed was more than 65°C rigor mortis does not develop in the body.
But the body becomes rigid due to reason other than rigor mortis.
Keep in mind rigidity and rigor mortis are not the same. Rigor mortis is a specific term for rigidity which is caused due to depletion of ATP after death.
Rigidity in burned dead body is not related to ATP delpletion (so no rigor mortis occurs in dead burn body) rather it occurs because of denaturation and coagulation of proteins that causes the bulkier muscles to contract and thus rigidity occurs.
Rigidity in burnt corpse is also called pugilistic attitude due to flexion of arms and legs.
Normally, the rigidity due to burn remains till decomposition starts but rigor mortis is temporary phenomena and body gets breakage of the rigor mortis after 12–24 hours.
Thus rigor mortis does not occur in burned body.