Wiring L14-20 Receptacle on generator to a 6-20 Plug
Correct.  The 6-20 plug has no neutral. It needs no neutral.  

You don't even need to use /4 flexible cord**.  You can use /3, hot-hot-ground or hot-(taped white)-ground, and there won't be a neutral wire to connect. 



** In flexible cord, grounds are counted in the number, unlike Romex/NM cable.