Got permission from the original author! Here's the message:
"您好,感谢您的来信,我一般不看b站私信,现在才注意到你的来信,作品许可问题您无需在意,收费与否也看您自己意愿就好" (Original Text)
"Hey, thanks for reaching out! I usually don't check Bilibili PMs, so just saw this. No worries about the licensing, and whether to charge is totally up to you." (Translate)
So no copyright worries, enjoy freely!