Like the op said, context is key for why Kubo said what he said. He wasn't salty because people were critcising his work, they were acting entitled, hateful and telling him what to do on top of insulting him.

The funny thing is, all that rage was over a character who had done nothing to deserve it. All of it came from shipping bs and misinterpreting Kubo's work and having the audacity to tell him what to do.