Their possible motivation for releasing code as open source is to allow some other Russian companies like VK Group to build their IP on it without having any deals with them directly. Just now VK Group announced their in-home game engine will be based on this code.
I can guess that Gaijin Entertainment had some contracts with companies in Russia, but after moving their business out of Russia it's now easiest way to fulfill the terms of some IP licensing deal.
Yandex is doing the same thing, but other way around by releasing more and more of their code as open source. It let them to cleanly trasfer IP from Russia-poisoned company to their Israel company.
I can guess that Gaijin Entertainment had some contracts with companies in Russia, but after moving their business out of Russia it's now easiest way to fulfill the terms of some IP licensing deal.
Yandex is doing the same thing, but other way around by releasing more and more of their code as open source. It let them to cleanly trasfer IP from Russia-poisoned company to their Israel company.