Why do you think so? The verb nousta ylös was already introduced in previous lessons in the meaning of "to get up" and mäki is being practised in this lesson so with this very simple sentence structure I find it an excellent way of combining the previously learnt material with the new one, discovering another meaning of the Finnish verb. And it just means "to go up" (=ascend) so not like a difficult concept either. Besides, skiing is a very popular sport in Finland.
This is a messy one yes going up is better but then one could use menen ylös mäkeä. However the current form seems incorrect as nousta is to lift or to raise so this translation seems a bit of s stretch, and also not a very good translation
Nousta is also to ascend. I think in this case the Finnish and the English match.
Darn autocorrect. :p
So I haven't been able to find more precise information on the perspective, but is that that in "Minä nousen ylös mäelle", "ylös" is actually an adverb adding a (somewhat redundant) information to "Minä nousen mäelle", while in "Minä nousen ylös mäkeä", "ylös mäkeä" is a prepositional phrase?