-most twitter personalities dont ever respond, never saw him respond to anyone tweeting him.Sometimes I read comments like these and tell myself it's a good thing for my patience that I try to read most posts in a monotone voice. Cause good god it would be so easy to naturally read this in a not at all flattering tone.
Anyway
1. Not every reader of MHA or his Twitter is American/Japanese or lives in either country.
2. Similarity with the system of another country does not remove notability from his statement. "This is how this works" is a viable principle behind a comment, even without "and how it's different from X" tagged onto it at the end.
3. Not gonna speak for others because I don't feel like assuming what is "common" knowledge for others regarding a country they've never been to or have no special relationship with, but I didn't know Japan had capital punishment. In 2018, capital punishment isn't in a place where you can safely assume it's in practice or not.
4. You use Twitter, as you're certainly not shy of reminding us periodically. Just ask him the question directly if you actually care for answers to them and understanding into his perspective.
-I dont know if you necessarily need to live in a country to have knowledge about a country. In this globalized world it is polite to know general knowledge about the people around you. And for a hot subject as capital punishment, it is rather notable that JPN is one of the major countries still with it including several other large countries like China and USA.
It is just unclear what he was trying to point out with this, as in his other posts he either mentions unique connections with name aspects or wordplay, or links/callbacks to previous things in the series, where here it feels outside of his modus operandi. Perhaps he was trying to make the implication that even in a future based superhero society, that capital punishment is still used as something for criminals?