It's not the first time I've heard this though. The point is that you lose some of the dynamics that are characteristic of slide playing.
"Slide guitar is so powerful, so expressive and so emotive, and to get the most out of the technique it really helps to use pickups skewed toward vintage output levels rather than modern higher gain pickups."
http://www.gibson.com/News-Lifestyle/Features/en-us/Setting-Your-Guitar-Up-For-Slide.aspx
It's not like here metal pickups are recommended:
http://www.seymourduncan.com/blog/the-tone-garage/great-pickups-for-lapsteel-guitars And well, the antiquity lap steel has a dcr of 9. I think the point is not to recommend super low output pickups but rather anything in the "vintage output" area.