Isn't さん optional?

I feel like my answer should've been accepted, if they wanted me to use さん, they should've added honorifics (Mr./Ms) or something right? Also isn't it only used if you don't know the person too well? If you're friendly is it omitted? What's the context here?

I feel like my answer should've been accepted, if they wanted me to use さん, they should've added honorifics (Mr./Ms) or something right? Also isn't it only used if you don't know the person too well? If you're friendly is it omitted? What's the context here?