The article, says that IBU was measured with a spectro-photometer. The 50 year old, standard test method, which doesn't reflect the perceived bitterness.
A study (by Maye), also listed on the excellent scottjanish site, describes why the IBU test, may have little meaning for dry hopped beers.
"Dry Hopping Effect on Bitterness and IBU Testing"
https://scottjanish.com/dry-hopping-effect-bitterness-ibu-testing/
summary:
"The main issue with the standard IBU test, is that although it can detect iso-alpha-acids, most of the alpha-acids and most of the humulinones, the IBU test cannot differentiate between them. This is an issue because we now know that humulinones are 66% as bitter as iso-alpha-acids, and alpha acids are about 10% as bitter as iso-alpha-acids, which means the IBU test isn’t accurately measuring the sensory bitterness of a beer based on the varying intensities of bitterness from the different hop bittering compounds. Also, about 12% of the IBU test result, comes from other hop compounds, perhaps non-bitter polyphenols.
Nowadays, high-performance liquid chromatography (HPLC) testing, allows for the separation and accurate measuring of the various hop acids in dry hopped beers. You can then calculate the bitterness, by adding the bitterness intensities of the different hop acids relative to iso-alpha acids."
For example, through HPLC testing, Maye was able to show that by dry hopping a considerable amount of iso-alpha-acids were removed from beer and replaced by less bitter humulinones and alpha-acids because the test could differentiate between the different hop acids."
Interesting findings, were that: dry hopping beers is likely to make beers with IBUs under 20 become more bitter, while those with IBUs above 30, become less bitter. And Humulinones introduced to beer via dry hopping imparts a “smoother” bitterness than iso-alpha-acids and are 66% as less bitter.
