Zener temperature derating
It looks like the Ta curve is consistent with the \$\Theta_{JA}\$ figure (Which doesn't seem to give mounting conditions.)

So at 90C per watt, and 150C max junction temperature and 25C ambient the junction can rise $$150\text{C} - 25\text{C} = 125\text{C}$$ Then $$MaxDissipation=125\text{C}/(90 \text{C/W})$$ gives ~1.4W .

The TL line says that IF you can keep the lead temperature below 75C, you can dissipate 5W all day.  Of course that may be easier said than done but the specs are consistent.  (In actual practice on a minimum pad PCB the lead temperature will quickly rise above 75C with 5W dissipation at Ta = 25C, so you would need some way to remove the heat from the device to maintain 5W capability.)

How they can call it a 5W diode is really all about marketing :)