Robert Fico, Donald Trump, Charlie Kirk, Solomon Pena, Fumio Kishida, Andriy Parubiy, Alejo Vidal-Quadras Roca, Shinzo Abe, Jair Bolsonaro...

All of them represents of right orientated politics and are a victims of assassination in recent years.

What does that have to do with US domestic terrorists, which is what the study concerned?

Trump is the only one on your list where it was a US domestic terrorist.

The shooter in that case seems to have been more right-aligned than left-aligned but there is nothing definitive. He donated $15 to Democrats when he was 17, but a few months later when he was 18 and registered to vote he registered as a Republican. A social media account that is thought to be associated with him had a lot of anti-immigration and antisemitic content.

The only two of those I know of were shot by far-right conservatives...