There still is Set Source and Set Dest, of course.
Opus on the PC originally also had Lock as Source and Lock as Dest, like the Amiga version, but I don't think anybody ever used it except by accident.
People definitely used to get confused because the active pane was not always the source pane which resulted in the source/focus being tied together in Opus 8 or 9 (I forget which).
As the button tooltips explain, the lock buttons lock the path to the file display, rather than locking the path from being changed at all.
Tab-lock stops tabs from changing paths. It's got no relationsship with source/dest.
You could make a button which ensures the left side is the source before opening the Sync panel. If the two locks are off then that should solve the whole problem, I think.
Similarly, when running external commands you can ignore source & dest and explicitly request the left and right paths instead, to ensure things are always a particular way around. Doing the same with the selected files -- rather than just the paths -- currently requires a slight hack but works very reliably.
When else do you need to use left vs right, rather than source vs dest?