alt Hacker NewsIn non-English texts it is not unusual to see English honorifics like Sir, Lord, Lady, Duke etc. or even Dr., Mr., Mrs.
Similarly, in English texts it is not unusual to see foreign honorifics besides the actual names.
It is quite frequent for someone who otherwise does not speak another language to address foreigners as they would be addressed in their own language in formal situations where politeness is expected, e.g. using Herr or Frau for a German, and so on, or using Takaichi-san or Takaichi-sama (more formal) instead of translating that to Mrs. Takaichi.
I think that when speaking about a prime minister, formal language is not inappropriate.
Trump is probably the most obvious chief of state whose name would look completely inappropriate in the context of using formal polite language, but this should have been an exception.
supplementing your points: Japanese seem to prefer dropping the honorifics when in English:
https://archive.ph/OI3S2
I've also heard that it's still common to address one's peers with -kun in parliament.
In GP's case it sounds rather quaint, but also endearing (like referring to a neighbor or a long lost schoolmate in the third person)
Pedantry: Takaichi is her father's surname. (I had to look that up, tbh)