You don't need a default route in this case. A route to the local subnet will always be set via the device that performed the DHCP query, so you will be able to reach the ESP. And since the ESP cannot route to any other networks, there is little benefit in having a default route via the ESP.
The only issue might be, if you try to send UDP broadcasts to 255.255.255.255 instead of the local broadcast IP. At least on linux, those packets are routed via the default route (again: I did not test what would happen in this case).
Also the documentation does not mention if the ESP will still set its IP as DNS server in the DHCP offer. If it does not, then disabling this option would hinder any attempts to have the ESP act as DNS server (e.g. for using a hostname, captive portals or the login page notification on android devices).