“When we update this prior in light of the Fermi observation, we find a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe (53%–99.6% and 39%–85% respectively). ’Where are they?’ — probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable.”
Did you even try to understand the paper? They rightfully point out the uncertainty in estimating even the order of magnitude in the Drake equation, then build a model on uncertain, if not completely speculative facts including the Kardashev civilization model, and the validity of the "Fermi observation," which is in fact being debated in this very thread.
https://arxiv.org/pdf/1806.02404.pdf
Done and done.