TVS diodes are designed to withstand high junction temperatures those they have much larger die area than the conventional zener diodes. So don't try to calculate \$T_j\$ of a TVS diode at surging time, using the conventional thermal equivalent circuit. Even you cannot use the transient power-temperature or thermal time resoponse calculations as in regulator zeners.
For steady-state temperature the junction temperature, and using the minimum recommended footprint will be:
Derating power dissipation @55°C = 0.682W, so
$$Δt=θ_jL \times P_d= 25°C/W \times 0.682W = 15.7°C$$
$$T_j = 55 + 15.7 = 70.7°C$$
Soldering footprind recommended by the manufacturer shown bellow on a FR-4 standard board
The big difference in junction temperature between the two mounted methods is that this diode package is designed to dissipate better from leads than the case.
With this new junction temperature, the power capability of the diode reduced to 420W for 1ms pulse, and the new brakedown voltage increased by 1.1 volt because TVS diodes temperature coefficient it is similar to Zener diodes.