Are you positive that the world can see this page? If I understand
correctly, if you turn off port forwarding then the HTTP page of the
router is displayed correct?

If this is the case, there should be an option in the UI to turn this
off, in the stock software for linksys routers there almost always is,
if I gather your meaning behind ddwrt correctly, it is a non-stock
software for the router. If this is the case, I'm not sure how.

Myself, I would also divert port 80 traffic to somewhere else, if a
web server is not on that network then I divert it to a non-existent IP.

Also, verify that your router isn't being smart, I know with some
routers if you visit your external IP from inside the network the
router sees that the packets are coming from the inside back to the
inside and gives you access to the main page - while if it really does
come from outside your lan, it does not.