This has caused a bit of a stir in the forums as some players believe this will send the wrong message. "Since the game's been released for 3-4 years or whatever," player Norrdec observes, "isn't going early access kinda... fraudulent?"
DEV Zero later clarifies: "The main reason for it is because we have no idea what to expect regarding player numbers and how the servers will take it. We need to warn people that there can be server issues, lag, etc., in the beginning and stuff can break badly. And right now on Steam, being early access is the best way to bring this to people's attention. Obviously we'll tell them this in the developer's notes box and that we've been out for years already."
Be sure to check out the entire thread for more on the Early Access plans and a proposed Gamma wipe in the near future.
[Thanks to Anon for the tip!]