I don't think so. Rainforest destruction and the impact on species like the Sumatran orangutan is what justifies their actions.
The statement you've quoted is more for the reader - perhaps it is trying to say that they have the popular support of hundreds of thousands of people, which if true is notable (not many popular movements are bigger than tiny when compared to the world's population). I think it also attempts to imply that 'we're winning' or 'you're not alone in supporting us'.
I don't think Greenpeace has any justified reason to be boarding that ship. They're lucky they weren't seen as potential pirates and shot at. I don't support Greenpeace's actions generally and think that they're counterproductive because they tend to draw attention to the protestors but not the reasons. Same with those idiots that harass the whalers. Neither group seems to offer viable alternatives to whatever they are protesting.
I think they're completely justified. Whether I support the action or not depends on its effectiveness in terms of promoting positive change, and I'm not sure of that. If they didn't think they were having a positive effect, they wouldn't bother.
So that justifies their actions? A tiny percentage of the world's population?