That seems very unlikely to me.

Based on https://en.wikipedia.org/wiki/Hungary%E2%80%93Russia_relations#Hungary_and_the_Russian_Federation: Hungary has mostly remained neutral-to-favorable to russia while remaining part of NATO. So I suspect they don't have as strict export control laws as other NATO countries.

But also: What would be the point of open sourcing your engine so that a russian company can work on it or use it? If you are going to be doing back channel exchange of money to outsource improvements then that is a REAL good way to get your assets seized. And the commit history will be a good indication that something hinky is happening. And if you are going to back channel it you just give them a private FTP server or something.

I am not huge on warthunder so I can't really speak to the technology. It looks like it has some good scale capabilities though. So I could see this as being the first step to try and pull an "Unreal Engine" and branch out that way. Although, odds are this will be closer to a Cryengine.

A more likely, but imo still not realistic, theory I've just thought of is that this was done to reduce intl intelligence services trying to snoop on Gaijin devs because they think could be classified stuff in their source code.