The highest CASE temperature mentioned is in a diagram on page 9 - and max is about 110 C.
The highest device temperature quoted, apart from soldering temperaures is 150 C at the top of page 2 - and this is JUNCTION temperature.
Ambient temperatures are relevant but not under designer's control.
The JUNCTION is the actual IC core, inside the chip.
Junction temperature relative to case is determined by power input and Rjc = thermal resistance junction to case.
BUT the datasheet seems to be bad in this respect and give only Rja (shown as Theta_ja) = resistance junction to air. You can see by using Rja whether you need a heatsink and whether that the datasheet should give Rjc. Looking at the figures:
Rja with 2 layer board = 27 C/W.
Device is said to be 93% efficient.
At 20W in 7% is dissipated at heat in IC = 1.4W.
At 27 C/w IC junction will rise above air by 27 C/W x 1.4 W ~+38 C rise.
Tj max = 150 C, so allowable ambient / air / board temperature is 150C - 38 = 112C.
If we use the 37 C/W Rja for single layer board we get differential rise junction to air of
37 x 1.4W =~ 52C rise over Tair, and max air/board temp of 150 - 52 = 98C.
Gven the various assumptions made, 98C is close to the specified 85C max - so we can see why they specify it. At 85C Tboard you can r un the IC at 20W power in and somewhat worse than 93% efficient and not (quite) exceed the 150C allowed max junction temperature.