Probably to incite discussion on the state of the Japanese economy.

It is not a "politician makes non-promise" story. It is actually a very credible claim because the Japanese government doesn't even need more tax income as they can lend freely.

The government can lend as much Yen as they want. The tax hike is something that can help them get inflation, which has been too low for very long.

Even if it is not an empty promise I still don't understand how this is on topic for HN

I called it a non-promise because, like every second time a politician says something, he appears to have carefully worded it such that he still change his mind later. Not because I don't believe him, I have no opinion on and no idea about Japanese internal economic policy. My argument is that the tl;dr here is "a politician said a thing", which is not usually notable enough for HN.

I agree. It's offtopic.