The thing is I am not sure if Igor was developing Nginx on his own time. IANAL as well, but I am not sure if it counts if part of the work was done from his Rambler workplace.
According to Russian laws it does not matter. Igor was hired by Rambler to do system administration job, he was never assigned any software development job - that means Igor owns the copyright. His ex-boss is confirming this (in Russian: https://roem.ru/12-12-2019/281134/rambler-nginx/#comment-292...)