The linked paper is too involved for me to really parse/grok, but I'm curious to know if the study(s) cited occurred before or after the introduction of skin-toned variations.

If there was only one colour available, and everyone knew there was only one option, would that lead people to think it was more neutral? Or, if the study(s) were post-variance introduction, people came to think the supposedly-neutral colour is 'actually' white.

Did the introduction of variations also introduce the idea of non-neutrality?

The paper was written in 2021, the skin-tone modifiers were added in 2015.