Ran into this, it's just unbelievably sad.
"I never properly grieved until this point" - yeah buddy, it seems like you never started. Everybody grieves in their own way, but this doesn't seem healthy.
It really just means a one-sided relationship with a fabricated personality. Celebrities being real people doesn't really factor into it too much since their actual personhood is irrelevant to the delusion - the person with the delusion has a relationship with the made up personality they see and maintain in their mind. And a chatbot personality is really no different, in this case, so the terminology fits, imo.