While I can certainly see that as an origin story, Wikipedia puts it as an offshoot of Aloha Friday - https://en.wikipedia.org/wiki/Casual_Friday / https://en.wikipedia.org/wiki/Aloha_shirt#Aloha_Friday - in the 60s.

My favorite origin story is one that mixes some truth with fantasy and a religious pantheon of the Casual Gods - https://everything2.com/user/m_turner/writeups/Casual+Gods