What about other display managers, like KDM or Entrance or XDM?
I answered that in the mail you replied to (but not the part you quoted) :-)
Originally, we were going to start GDM early, but have gotten good
results without that, so we aren't going to do it for now.
We are going to have to do some integration work with GDM to ensure a
smooth transition between plymouth and GDM, but if that integration
work is lacking in other display managers it should just result in
flicker, not a fatal problem.