Ah I see.
When I say “progressive” I only mean the ~ている form of verbs. I agree that one interpretation of 日本に行っている is “is in the state of being gone to Japan” (or more naturally in english, “is in Japan” while implying he went there). But couldn’t it also be interpreted as “is currently in the process of going to japan”? (more naturally in english “is going to Japan (as we speak)”).
For the sentence “彼は日本に行っている”, deepl gives me three possible translations: “He is going to Japan”, “He’s gone to Japan” and “He goes to Japan”. While I wouldn’t consider any automated translator (AI or not) to be authoritative, this does match my beginner’s understanding the ~ている form of verbs, does this make sense to you too?
If I’m right (which I’m not sure I am), that would mean that both bunpro’s translation and your proposed one are correct, they’re just two possible interpretations of a sentence that lacks a bit of context.