Not to be a nitpicker, but a quick google search tells me it is impossible to make a diamond by compressing coal. Diamonds are pretty much pure carbon, while even the purest of coals have a lot of other elements besides carbon, which would prevent the structures of a diamond from forming. If he used graphite, which is also pure carbon, it would be closer, but even then there are quite a few other conditions including very high temperatures that are required.
So, the story suggests he only used body enhancement to apply pressure to coal and formed a diamond this way. But that's impossible and given his isekai knowledge not that creative. So it would actually be more interesting if he used magic to simulate the high-pressure, high-temperature conditions, while also removing impurities from coal, to form a diamond this way.