Having 1M batteries being retired will be long after 2020.. The batteries should have 10-15 years lifetime and there were only a few 100 in 2010..
I suspect that they should have stated "modules" or "cells" rather than batteries. I'll also assume they are counting cells from all EVs and not just Volts. Will secondary markets support pricing at something near 80% of replacement price? i.e. if 2020 prices are $200/kWh will used cells with 80% remaining capacity command $160/kWh ? If so, this could really impact the TCO of a EV in a very positive fashion.