Ehrlich is simply wrong when he says "[the study] shows without any significant doubt that we are now entering the sixth great mass extinction event"; that would require 75% or more of all species to disappear; the paper only looks at a small subset of species, those most affected by humans.
I wondered if the extinction record could be used as evidence of past intelligent species on Earth. If there were any kind of prior top-shelf minds spreading across the globe one unmistakable evidence would be in how they shaped the foreign ecology they spread into.
Tools and buildings may not survive deep geological time. We may not even recognize something created by a very different kind of intelligence as a tool. Ancient hearth and midden piles on the Amazon are mined today for rich soil and charcoal but really hide the enormous human presence on the river that disappeared overnight to mostly likely European diseases. It took a surprising amount of time for someone to figure out that stuff came from us just a few centuries ago. But a sudden biodiversity decrease without a volcanic or meteoric crater and a change in seafloor sediments might be a smoking gun.
I would think humanity is almost a best case for leaving behind a detectable legacy. Outside of a pathogen we are the ultimate bad neighbors for a tasty species. Humans are aggressive territorial omnivores with very poor hygiene as a group. We may leave behind just enough trash to tell someone we were here long after our radio and radar signals are lost to the background noise. But what about our paleontology record?
If we cannot even kill most of the vertebrate species then that argues against using the extinction record to track prior intelligence that may have arose here. A technological species that develops green ethics before hitting the shoot-it-if-it-moves level may leave little change at the level we can detect.