The plot thickens ...
In this article about
Frank Mason Robinson it details Robinson's 20 year career at Coca Cola and how his logo was "based on Spencerian Script."
However, I suspect, this article also falls victim to the "Spencerian Script" basis myth as well. What I think really happened was Robinson, who was a bookkeeper by trade would have been familiar with both Spencerian and Roundhand. I think he crafted the logo, "borrowing" the Madarasz swoosh and lettering an amalgam of Roundhand and Spencerian. Perhaps because he was not a penman, it looked to him like Spencerian. (Or someone later on down the line gave it that attribute.) No one thus far has established where the idea he based it on Spencerian originated other than a century later speculation.
This article mentions Robinson as an "adept utilizer of Spencerian Script" and as a "skilled calligrapher." Looking at the original logo - given the placement of the shades - it was lettered, not written. And "adept" would not be my first adjective.
Even shaded Spencerian looks vastly different. I see no characteristics of Spencerian in that original logo. If someone else does, please correct me.