There is an interesting question raised in this post, but it is not addressed in a meaningful way.
If we wanted to see the trend for millenials, we wouldn't compare a landslide (2008) with a fairly close election (2020)
What I would do is look at the millenial vote relative to every else for the last 6 or so elections and see if there is a trend.
Or maybe its extremely telling of where the Democrat party has gone since Obama ran in 2008.
I was a self identified Democrat then when I pulled the lever for him.