Not a lawyer, but surely something can be both open source and trademarked? As far as I understand those are unrelated concepts. In this context, the “source” is the SVG markup.
The issue is how you use it. Whether the icon SVG markup is open source isn't very relevant if you're not allowed to use it. If you want to personally mess around with it and not use it in any commercial context, no-one is going to care.
