Can 一人 not mean “alone”? It flat out rejects it and insists it only means “one person”. This is after linking WaniKani (which insists the opposite, that it means alone first and foremost.)
I would personally say that the primary meaning is “one person”. It can mean “alone” or other similar things, especially when used in certain patterns (e.g., 一人で). There is also the word/spelling 独り (e.g., 独りぼっち).
I just asked my colleague and she said out of context she would always define it primarily as “one person” however the usage that would mean “alone” in English is not mentally separate for her (she doesn’t know English so that conclusion is just from me explaining the situation to her).
The dictionary also has this to say on the matter:
一般に、数に重点を置いて「一人」、孤独・独立・単独などに重点を置いて「独り」と使い分ける。
(rough translation)
Generally, the difference in usage is that 一人 emphasises the number (of people) and 独り emphasises solitude, independence, etc (this is the “alone” definition).
Whether this means “alone” should be accepted on Bunpro or not is not for me to say but hopefully that clears up a bit how the word is generally used and thought of. I am guessing Wanikani is strict on this because it uses the “alone” meaning for its mnemonic system later down the road or something? Honestly no idea as I am not a user.
Hopefully that helped! Let me know if you have a follow up question.
Thanks so much for the response, that makes sense!