A true progressive scan DVD player gets the picture information off the disk and formats it into a progressive scan video signal without converting it from digital to analog until the last moment when it is ready to go out the output jacks.
Component video is also analog.
The progressive player but not the external doubler can sense flags in the video data that tell which even fields match which odd fields. The difference in picture quality between using a good progressive scan DVD player versus using an ordinary player together with a very good line doubler and component video connections is not large. Progressive scan DVD players with not so good de-interlacing do exist. It is a buyer beware situation. Even if you understand de-interlacing and progressive scan thoroughly, it is hard to figure out what methods a player uses because each manufacturer has its own trade secrets.