You are right. The not-quite-RC4 implementation is mentioned in the Talos post, and it is originating from BlackEnergy. Talos is referencing a US-CERT report of APT28/29[0], which links an F-Secure whitepaper on APTs using "crimeware"[1]:

BlackEnergy is a toolkit that has been used for years by various criminal outfits. In the summer of 2014, we noted that certain samples of BlackEnergy malware began targeting Ukranian government organizations for information harvesting. These samples were identified as being the work of one group, referred to in this document as “Quedagh”, which has a history of targeting political organizations.

The only way I see how anybody could conclude from "APT uses a black market toolkit" to "Anybody using this toolkit is that APT" is: clickbait.

[0] https://www.us-cert.gov/sites/default/files/publications/AR-...

[1] https://www.f-secure.com/documents/996508/1030745/blackenerg...