You are absolutely right, this is a bit sloppy. The premine was a little below 25 billion. It is lost somewhere in the thread above, but the formal premine (which you can find in the source code on github) was "only" 24 billion. Then we also did a small in-class experiment (about 1 week of mining which should give 2400 blocks or 24 million SMLY), so the full premine was 24 billion plus a few million mined during the experiment. We really wanted to do the in-class experiment with rewards before launching, just to know whether we were missing something.

I guess I could have counted the exact number, but it felt simpler to just talk about 50% even though it is actually slightly less (more like 48%).

Sorry for the confusion.