It's only one train a day, so it doesn't need to fulfil that much of a market (i.e. it's enough to cover just the people who are in central London and able to get there by 7.00). Also, it stops at Lyons and Avignon on the way, Marseille is an addition to the service, not the main purpose:

> In fact, a few days ago, getting the train to the UK, only a few passengers boarded at Marseille. But the Eurostar filled up nicely at Avignon and Lyon where it stops as it heads north.

Good point about the change, though. That really renders the headline untrue. It's not a direct service.