Hydrogen gas combines with oxygen gas under higher pressures and temperatures.
Chemical exothermic reactions produce the vast majority of the heat of evolving stars. This is why Jupiter, Saturn, Neptune and even Uranus radiate as much as if not more energy they receive from the Sun, and will continue to do so as they form the "Earths" in their interiors. We have ample evidence of these chemical combination reactions having taken place all over the Earth:
ALL of these compounds (except for the native elements) were formed in fact via exothermic reactions, as heat would be released from bond formation. You don't get chemical bonding for free, the gravitational potential of the collapsing star provides the energy for these vast chemical combination reactions to occur inside of the star. The compounds listed constitute the 7 major mineral groups.
Saturn’s excess heat is generated by the precipitation of helium into its metallic hydrogen core.
With statements like this it becomes obvious they have ignored ALL chemistry, and the fact that if they are to have a cohesive "planet formation" model, they need to include how the chemicals on the Earth formed. Just ignoring them doesn't make them go away. (I have also noticed the typo in the graph, carbon is C, Gold is Au, Silver is Ag)