Word boundaries in auto-completions and other functions are hard coded and not configurable (The linked issue requests them to be more adjustable). Auto-completions has seen several changes to fix quite a number of issues with regards to non-word characters. All those changes or new heuristics are hard coded as well. So there is no way for syntax authors or end users to adjust things for certain syntaxes - all of them coming with their own special needs.
The main issue might be - just guessing - periods to be used as accessors in many popular languages, so new AC breaks words when seeing them.
As AC probably won’t go the way back to make it work like in ST3 as it breaks many other things, the only chance is to request it to respect
word_separators setting. That’s the point.