If nginx has dubious copyright, you cannot fork the code, since not being open source, it is legally not-forkable. Yes, you could do a complete rewrite, but that would require doing so in a "clean room" fashion, which would be very difficult.
In any Western court an entity who forked it, would win a copyright lawsuit if Rambler sued. You cannot claim it was your proprietary code all of a sudden after it's been openly available for 15 years.
