This is not that surprising. The people who took online tests in 2006 may have been earlier adopters, and and thus slightly smarter than later test takers. Second, it's impossible to make an extrapolation about online tests ad being applicable to IQs a century ago.
As it's said in the paper:
As the annual sample sizes have increased in more recent years and the SAPA Project has discussed in more public outlets such social media (mistressredditor, 2013; Murray, 2017; SAPAPsych, 2014a, SAPAPsych, 2014b) or online articles (Guarino, 2018), it's plausible that newer annual samples are more “average” or normal representation than those recruited during the SAPA Project's former years.
I don't think it's supposable to conclude that flynn effect is reversing