There has been some recent changes to the license of wpscan, which is not obvious from reading the article.

Initially, wpscan was licensed purely under gplv3.

On 26 Sep 2014, the 2.5 release of wpscan changed the license to a self-described dual-license scheme where noncommercial could still use GPLv3, and commercial use had to use a commercial license. GPLv3 permits you to remove any addition constraints, so license text and projects intent clearly conflicts.

Yesterday the license was changed again to a newly created license called WPScan Public Source License. It is similar to Creative Commons Non-Commercial which is commonly defined as non-free, non-open-source license. It is incompatible with all version of GPL.