I don't really think the territorial claim on the land where the extrajudicial assassination happens is very relevant to the legal fact of the matter.
We don't say that the FSB attempting to execute Skripal in a UK shopping mall doesn't count because it was in the UK. Murder is murder.
The claim that the IC would assassinate Gellman in New York was made by someone who used to be an actual CIA operative and went through their training.
It absolutely doesn't matter where it is. All that means is that the means of murder will change. Instead of being an airstrikes it might be someone off the street paid for the hit or an operative killing them or poisoning them. In both cases, you'd have no way to know. The only places you'd be able to know are in foreign countries where risks of capture and death are much higher and where investigations can't be silenced.
It really diminishes your point when that is compared to an airstrike on foreign soil.