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That's a really bad description in that article of a really profund achievement. It glosses over rest mass, momentum and it shouldn't simplify mass can be converted into enegry, it would have been better to say mass and energy are equivalent - they are two sides (or properties) of the one coin.
Its imprecise to say the mass of a particle with energy is zero - as mass and energy share an equivalence and the energy isn't zero. Say the rest mass is zero. As I think you'd find this study shows a gluon has momentum. Really you need to think matter, energy and local spacetime curvature (or average energy density) when you are trying to describe what is happening and what it shows. They would have been slight better to describe matter simply as trapped or confined energy (even that is imprecise and wrong) within a volume of spacetime.
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