To clone UNIX, in whatever form they had to have had access to it in first place.
Which was with the tapes that flown out of Bell Labs into several universities and the commercial licenses that were almost gratis, given the price difference to other OSes.
https://www.bell-labs.com/usr/dmr/www/licenses.html
People keep reinventing history as if UNIX won based on merit, when it was licensing that made all the difference.
Like the fairy tale that before C no one ever did a high level systems programming language.