Given that Vulcanus' ashland trees produce carbon in regular Factorio, it would be strange if the trees grown by the agricultural tower gave coal instead of carbon. And I personally like the idea of farming those tress with an agri tower as a source of resources on Vulcanus, otherwise it would just be everything from lava in one form or another.
The coal + acid > carbon recipe still exists, but you are correct you're unlikely to use it in seablock. While tungsten carbide is a bit simpler since you dodge making carbon, the complexity is added back in other ways. You make sulfuric acid from carbon + calcite instead of getting it "free" from pumpjacks, and you'll need to go "backwards" and make quite a bit of coal from carbon in order to make the oil/plastic you need for things like rocket launches.
If you feel like the Vulcanus production chain is lacking, I'm certainly open to other ideas to spice it up a bit.
