What does a junction do inside a MOSFET?
I think you're being unduly picky- it's only a datasheet- everyone knows what it means, like 'dialing' a phone with a touch screen. (Almost) all semiconductors have junctions somewhere (maybe some silicon-on-sapphire or similar exotic types do not), and junction leakage is an important specification in junction-isolated semiconductors. 

Even in a formal document such as IEC 60747 they refer to the virtual channel temperature of a MOSFET, but the symbol used is 'j'!