Uhh, vanilla ender chest works the way I described so it's not like it is impossible to code. Just a little change to what happens when chest is accessed.
And I don't have problems with thieves, I'm just suggesting a helpful change imo. I don't see a point in a personal network of ender chests if other people can access it. 4096 codes for the whole server won't run out even without the personal chests.
Saying it is a coding limitation is not saying it is not possible only that the mod maker coded it that way. If you want to suggest this you would be better doing it to the mod maker themselves.
And the first vanilla enderchest worked the same way. One inventory for everyone it was later patched to work as it does now. This mod actually predates the vanilla version so was coded both before ender chests were even a thing and way before they fixed the everyone shares the inventory issue.