First of all, the point of my comment was to point out bad argumentation not prove the bug, so not having the time find sources when I made that comment was not my purpose.
I still stand by my comment that using a patch note 10-15 years later that says “previously” as a source is not a legitimate argument.
You found better information as needed.
All archived discussion I can find about mount speed, carrot on a stick, and mithril spurs state they were reported as multiplicative values by the addons that measured them then. Could be AddOns using the wrong formula, or it could be true.
Regardless, this certainly meets the threshold to be taken in as a supported ticket and potential bug in their system.