There are two separate equations here, one is the slope of the mountain, which is a function of x and a value of y and one is the path of the sled, which is a function of t, with a value of (x,y)
But even ignoring that, nowhere does it define the relationship between t and x. What I assume they mean but don't say is that x=t. That's arbitrary.
Also I assume you mean the path of the sled is (t, f(t)) where f is the function defining the slope. If the path of the sled had a value of (x,y) it would be stationary :)
Perhaps "the shape of a path down a mountain" would make everyone happy.
If they'd just said the above, and I think not shown the value of t, it would have been a lot clearer, at least to me.
If in later levels they show formula(s) using gravity, and perhaps show the skier on different planets, with different values of G, then I can see the value in showing the value of t. But as it is, I think it's confusing.