Correct: it's a self-selecting sample of people who are attracted by the Good Country and/or the Global Vote, so the results have little value as a statistical representation of the views of the world's population. No surprises there, because the Global Vote is not research.
The Global Vote is a way for people to participate symbolically in the elections of other countries, and to learn interactively about the politics of those countries. They do this because they understand that people and places are connected in many ways these days, and because they care about what goes on around the world.
And that's why we don't collect lots of demographic information about our voters or publish a detailed breakdown of who voted for which candidate in which country. The Global Vote doesn't exist in order to find out how a particular candidate would perform if the vote was international and it certainly doesn't exist in order to promote the interests of any candidate: it exists because so many people want to learn more and feel more involved in the way political decisions are made around the world, and to send out a message that we're all affected by those decisions.
One day, it might be interesting to start collecting and publishing more detailed analysis on how people vote on the Global Vote, but it will only make sense when we regularly have many millions of people participating. For now, the point isn't the numbers, but the taking part.