I agree that the gap in inherent capabilities from 4th to 5th generation is larger than from 5th to 6th generation, but "complementary" here is more about being more cost-effective in their respective roles than about the capability gap. For example, perhaps the lack of stealth makes 4th gens better suited to being missile trucks. Perhaps they are also cheaper to operate against lower-end opponents when carrying out many of the "dirtier" tasks.
The point regarding 5th and 6th gen aircraft is that, for future procurement, basically every role that these additional 5th gen aircrqaft could do could be done better (and more cost-effectively) by 6th gens (by this point, we'd be looking at almost 1,000 J-20s, if not more, plus several hundred J-35s). So the argument is to simply procure the 6th gen aircraft instead.