Good point! I just did few tests and this is the summary: In CP I have configured Primary DNS server, in ipassignment.conf there is only my username and an IP. When I connect I got the right IP address and the DNS which is configured in the SmartConsole, so I believe this exception is a bit confusing and maybe it should means If you have configured DNS, WINS, DNS suffix in ipassignment.conf, the following configuration is not applied. As far as I know in SmartConsole is the only place where you can set lease time, in ipassignment.conf there is no such option explained, so I believe this option applies to every configuration.
During my tests with the customer last year with R80.10, we have set the lease duration to 10 minutes and a reserved IP address for a user in ipassignment.conf, connect to VPN with a client, receive the right IP and then cut the connectivity to Internet; After 2-3 minutes connecting again to the VPN and receive the next free IP address from the pool of Office mode addresses (not the one configured in ipassignment.conf because the lease time is not expired and the old one is not free); Disconnect and after another approx 10 minutes (the total minutes are more than the lease time configured) connect again and receive the right IP from ipassignment.conf.
These are my observation and it works more than an year and I did not receive any complains about this.
It will be good if any other did something similar to share his experience with ipassignment.conf
Do some tests in a test environment and see how it works without touching a live infrastructure.