Reddit was started as an experiment in free speech.
I recall Alex coming on Slashdot a lot to promote Reddit when he first launched it. "An experiment in free speech" was not anything I recall being discussed. I also remember him posting on Slashdot while still developing reddit.
What I recall, is promotion of a general interest platform that was more open than Slashdot (unlimited moderations for all!) and less susceptible to vote brigading than Digg.
It was while ago, so I may be a bit foggy on the specifics.