This survey didn't see an exodus among young voters from the Democrats. They favor Biden by 20 points here.
That's significantly different than what Pew's National Public Opinion Reference Survey said the other day.
The ATP has a much larger sample size than the NPORS and a different methodology.
I don't know where the truth lies, but I'm going to assume there hasn't been some sort of once in a lifetime shift among young voters until it actually happens in an election. For years now young voters have been overwhelmingly Democratic.