The laws of armed conflict are in fact actual literal laws (largely set down by the Hague and Geneva Conventions), so legality is absolutely the question.
In addition, there are two meanings to the phrase “act of war”, and you’re equivocating between them here. One meaning is an action to provoke armed conflict, and the other is an action that occurs during an armed conflict. The material point of the scholar’s contribution is that in the armed conflict between Russia and Ukraine, it is a permissible action for Russia to divert another State’s scout aircraft who is assisting Ukraine - i.e. it is an act of war in the second sense. This does not make it a Russian provocation towards the US - i.e. it is not an act of war in the first sense.
I am not equivocating between the two at all. I’m stating it’s very clearly the latter and because it’s clearly the latter, illegality is not a question. It’s obviously legal for Russia to take action against military assets that are participating in military action against it.
The whole legality question is a red herring, as your comment clearly articulates.
The US doesn't abide by those laws though so our government would be hard-pressed to use them as justification for anything. See the Hague Invasion Act for instance, we will literally invade the Netherlands if they try to hold any US citizens to account under those "laws".
In addition, there are two meanings to the phrase “act of war”, and you’re equivocating between them here. One meaning is an action to provoke armed conflict, and the other is an action that occurs during an armed conflict. The material point of the scholar’s contribution is that in the armed conflict between Russia and Ukraine, it is a permissible action for Russia to divert another State’s scout aircraft who is assisting Ukraine - i.e. it is an act of war in the second sense. This does not make it a Russian provocation towards the US - i.e. it is not an act of war in the first sense.