The assets themselves will not be open sourced, so if the project, including it's assets are open source, these assets could not be included. But if a project has it's code open source but it's art is not, which I believe is sometimes the case, then it would be fine to use these assets. So any one person who contributes to that project can buy the assets and put them in there.
My intention with the license was that if you buy the assets you can use them for absolutely whatever you want as long as you aren't selling or giving the assets themselves away to be used by other people in their own projects, who have not bought them.
I hope that makes sense and answers your question. I'll try and update the text with more specifics when I update the asset.