Wow! I just checked Eurostar for August 22nd (a Saturday) from London to Brussels and there were only 2 tickets left for the morning trains at the standard fare. I had no idea I'd have to book so early, but was glad I did. Only $59. per person!
Four months ahead is a good time to book; all departures should be open for booking. I'm pretty sure that note meant only 2 tickets left at the low fare of $59, not only 2 left in the whole Standard-class seating section. If the whole train were approaching sell-out, you'd be paying one of the higher fares (Standard class up to $300).
Yes- I didn't consider the higher prices seats for such a short ride. And they are considerably higher!