No they aren't. They are two different things. Two different skill sets, two completely different ways of getting down the mountain. The only thing similar is that you get down the same hill and even that can have its differences as to just how its done and where its done. They are not at all the same sport imo. That's not to suggest either being any wrong or right but only to say they are entirely two different things other than being on/in many the same places..
In the sport of rowing (my former passion), there are two disciplines.
There is "sweeping" where the athlete has one oar. To take a stroke the athlete pulls from the side.
There is "sculling" where the athlete has two oars. To take a stroke the athlete pulls from the center facing forward.
In either discipline, the athletes are in the same lanes on the same courses. There are many sweepers who can not scull and there are many scullers that can not sweep.
Similarly,
There is "snowboarding" where the athlete has one plank. To slide down the hill the athlete stands from the side.
Then there is skiing where the athlete has two planks. To slide down the hill the athlete stands to the center facing forward.
In skiing and snowboarding, the athletes are ride the same lifts on the same hills. Some skiers cannot snowboard and some snowboarders cannot ski. However, like in rowing, they are the same sport.