How is trading or investing a skill? Skill is something, that you do frequently and master it in a level that you do that particular job in a professional manner, quickly and efficiently. Let's take a carpenter as an example. One can't be a good carpenter without regularly doing this job and without involving in a lot of different and difficult cases, you can't repair wooden objects with any luck, you need skills, a good mind-muscle connection too while trader most likely depends on luck.
I would say that trading/investing is a combination of skill and luck. We can call it a skill when you do research and find good investment options but then here comes the luck, if market goes against you, there is nothing you can do to fix it while in case of carpenter, you totally depend on your skill, luck can't solve the problem.