Unfortunately I don't see any change in behavior when I enable or disable the "Send All Traffic" option.

Just to provide more details, this is in short the behavior that I see in every non-iPhone VPN client I have used with my home VPN Server (SoftEther, FYI):
If I configure the server to push to clients routing table changes (defined by me server side), such changes will be added to the client routing table as expected.
If I configure the server NOT to push any route changes to clients, clients will simply retain their original routing table, also as expected.

In the case of the iPhone VPN client it seems to me (no way to find out, apparently, unless I find an app to show me) that
- it drops entirely its own original routing table
- it creates a new routing table entirely, which routes all traffic through the VPN tunnel AND, also, fortunately, includes all additional routes I push from server side, if I do push them.