On WaniKani: that’s correct. In their FAQ (I think?) they link out to an article on Tofugu (aiui run by the same people) which gives you a couple good anki decks to learn hiragana and katakana. I started wanikani without knowing either, and found it manageable at the start by referring back to a hiragana chart. At some point I went through the decks, and after about two weeks I could read hiragana well enough to leave them behind.

Certainly not a complete resource for learning the language, but very effective for learning (to read) the kanji.