The only thing I notice in their prediction is we may be at the same point now as we were in apx. 1935 red arrow fixing to eclipse the 1930 green arrow peak...except we are now 2017 red arrow fixing to eclipse the 1970 peak with today's peak on shale being unpredictable it could continue to produce oil way above where we are today just like the production went a great big deal higher than the 1930 peak.
Shale production could go to say...30 million bpd. You never know. when it tails off like these guy predict it could be the reduction in production would settle out at 15 million bpd. you just never know. While their prediction of a tail off in production may prove out we may be dealing with a hugely larger amount of oil production than today or way more than anyone can imagine.
They just have no way of knowing how big the shale peak will be just like they didn't know in 1935 what the peak in 1975 would be.
They used to holler back in the '70's about peak oil...turns out they were wrong. They had no way of knowing what laid beneath or as my geophysicist friend said...who knew the cap rock we thought was junk was what was holding all the oil.