Actually, it's the meaning of the word in Greek:

> "what is regarded as true or correct," from Late Latin orthodoxus, from Greek orthodoxos "having the right opinion," from orthos "right, true, straight" + doxa "opinion, praise".

(https://www.etymonline.com/word/orthodox)

But, when referring to dual-pane file managers, it's probably a mix of both meanings ("one true way" and "old-fashioned").