Agreed!
I was a 21.0x guy who ran 10.8x in the 100m. I ran the 100m over and over again, and never really progressed, but consistantly ran 46.high/47.low splits in the 4x400m. I only ran the 400m open 3 times in competition in college and they were far better times than my 100m times.
It may be that you’re looking at the wrong distance for improvement. A couple more 400s might give you the strength to drop closer to the 21.0 mark.