Lol It is one of the funny tidbits I noticed too. In the first published version Ilya traveled via plane in chapter 1, but in the English official chapter 1 Ilya travels via boat. So the author basically re-connect the airplane entirely lol.
Ret-conned*
Might be because Hanu is not as technologically advance as Ilya home country. They haven't shared the technology with Hanu
Because Hanu isn't a city of technology and they don't have Chopper or whatever it's called they only have ship
In the first chapter, Ilya arrives to Hanu via a CHONKY, maybe even futuristic ass plane.
And yet in recent chapters, I guess author-sensei has solidified their world-building more and decided to de-modernize because Ilya traveled back to Romeridall via an old-gen sailboat lol (or the airports were destroyed during the Civil War?) Just thought that was a funny continuity (?) tidbit (＾▽＾)