Learn Finnish: Harju on niin pitkä kuin jyrkkäkin. - The ridge is both long and steep.

In the formal structure niin ... kuin ... -kin (both ... and), the suffix -kin (meaning "also" or "too") is a mandatory part of the pairing. It always attaches to the very end of the second item being linked, which in this sentence is the adjective jyrkkä (steep).

Doesn't niin ... kuin usually mean "as ... as" for comparisons? How do I tell the difference?

The deciding factor is that -kin suffix! A straight comparison would be something like Harju on niin pitkä kuin luulin (The ridge is as long as I thought). When you see the specific pattern niin [word 1] kuin [word 2]-kin, the -kin shifts the meaning from a comparison to the inclusive "both [word 1] and [word 2]".

Can I just use sekä pitkä että jyrkkä instead?

Yes, absolutely. sekä ... että ... is the standard way to say "both ... and ..." in everyday Finnish. The niin ... kuin ... -kin structure used here means exactly the same thing but is more formal and stylistic. You will encounter it frequently in journalism, essays, and literature.

If these adjectives needed to be in a different case, where would the -kin suffix go?

The -kin suffix is an enclitic particle, which means it always attaches at the absolute end of the word, after any case or plural endings. For example, if you were using the illative case, it would become niin pitkään kuin jyrkkäänkin (into [a ridge that is] both long and steep).