As stated in the article the mass fraction of phosphorous (P on the Periodic Chart of Elements) is used as a surrogate for ZDDP content.

No API certified engine oil that I'm aware of ever exceeded much over 1200 ppm P, and more can be detrimental due to long term corrosion mechanisms that have been observed at elevated ZDDP levels. Most "boutique" oils that I've seen are NOT API certified or carry an obsolete classification.

Back before there were limits on P (which began with SL), the P concentration of typical dual-rated (both spark and compression ignition engines) engine oils was in the range about 1200+ ppm. The limit on SN is 800 ppm - about a one-third reduction. The CJ-4 limit of 1200 ppm, which is only a marginal reduction from typical CI-4s, which had no limit.

Duke