They were referring to modern engines with roller cams, both new and broken-in; and vintage engines that are broken-in. All I can say in summary that if one has a a newly rebuilt (or crate) flat tappet engine, particularly high perf., high rpm, with high valve spring pressures, I would look to make break-in oil have at least 1200 ppm P, maybe higher (but NOT higher than 1800-2000!!), with either a pure ZDP additive or a high ZDP oil, and running oil at least 1000 ppm P.
They mentioned that P levels may drop in the future down to 500 or even lower. The Euro 4/5 ACEA E6 diesel truck oils have an upper limit of 800 ppm P.
Other SAE articles I have downloaded point out that ZDP has antiwear effects in locations other than the tappets: rings, rocker arms among others.
The factual basis for their conclusions were other people's studies with oils of varying ZDP content on camshafts and tappets, complete with pictures of destroyed cam lobes and solid lifters. It was a "meta-study", a compilation of others' primary studies.
Charlie