Your wiki link doesn’t list fentanyl in that section:

In March 2013, the site had 10,000 products for sale by vendors, 70% of which were drugs.[19][79] Drugs were grouped under the headings stimulants, psychedelics, prescription, precursors, other, opioids, ecstasy, dissociatives, and steroids/PEDs.[15][14][80][81] Fake driver’s licenses were also offered for sale.[82] The site’s terms of service prohibited the sale of certain items. When the Silk Road marketplace first began, the creator and administrators instituted terms of service that prohibited the sale of anything whose purpose was to “harm or defraud”.[14][83] This included child pornography, stolen credit cards, assassinations, and weapons of any type.

The bans look pretty explicit, and fentanyl isn’t one of them. Fentanyl is a wildly powerful opioid, which silk road seemed glad to sell. The site specifically allowed the sale of thousands of opiates of varying potency.

Your second link discusses how known fentanyl sellers were traced to silk road drug sellers bitcoin wallets:

This is likely not a new trend, and numerous vendors possibly also deal with other drug substances alongside fentanyl. Vendor clusters with Bitcoin payments to fentanyl suppliers have also been identified interacting with dark web marketplaces that have since gone offline, such as Silk Road and Hydra.

These links dont support that fentanyl was directly banned on Silk road. They actually show that fentanyl was likely commonly sold on Silk road in one form or another.