I am pretty sure they were aiming to reach that maximum since the beginning anyways.
Exactly. They just want some publicity so that people use Chrome. What better way than encouraging people to open lots of Chrome tabs?
It seems you need a Google account to be able to send your tabs, so it does not seem that they were aiming solely to publicize Chrome.
Probably they boosted-up the Google's account registration and post-registration page just before announcing the campaign.