@nprofile1q... Iran supplying Russia with military hardware through the Caspian Sea (and vice versa) is well documented over many years since Russia’s occupation of Crimea and eventual full invasion. Here’s one report from 2024 at the ISW. The Caspian is the most direct and safe link between Iran and Russia.
At least it was until Ukraine developed the capability to reach these distances.
https://understandingwar.org/research/middle-east/iran-update-september-9-2024/AlJazeera reports that this was a convoy escorted by Russian naval ships. So while we should double check the military cargo, not even Iran is disputing that it was military cargo. As far as international law, this would be fully justified.
https://aje.news/be7vlb