When this first was announced it was supposed to answer some of the questions brought up from the start of alien 3. Like how the egg got on the sulaco etc. So if Ripley entered and exited a tube I can see how this could still tie in to the timeline correctly.
After ALIENS she knows the company is out to screw her and kill anyone in the way to get their hands on an alien. After she kills the queen and gets everyone else tucked in she knows they are going to be pissed about blowing up Hadley's hope so she gets out of the tube to screw with the Sulaco's beacon so they cant be tracked as easily. (The ALIENS tech manual mentions that the Sulaco disappeared after what happened on Hadley's Hope and was made into a ghost ship legend by the marines.)
Ripley does whatever she was going to do and the new marines show up. They have already checked out the planet and whatever else they were doing in this game and had brought back some aliens and an egg or two with them for research. The bugs escape, mayhem ensues, the marines get whacked killing them off. Ripley is again thee lone survivor, thinking she is A-OK she goes back into the tube not seeing the last egg tucked away which opens on the start of Alien3.