I don't think the original OP implied that intelligence is a skill, but rather a capability like you said. Just like your capability to do physical work is not

I don't think the original OP implied that intelligence is a skill, but rather a capability like you said. Just like your capability to do physical work is not a skill but an ability dependent on your muscles which depend on how and how often you train em. It's the same with intelligence, if you train and exercise your intellectual muscle (brain), your capability to do intellectual work/exercises increases.