Hard core AOE damage there.

The point of this build (which I guess you didn't read it?) is dealing tons of retaliation and AoE magic damage. So it is a fairly nonstandard build for Rammus, I think, based on the other guides I have read. I never see anyone else build AP and magic penetration onto Rammus, but in my experience it works pretty well.

a build for only team against physical dps..?
i really don't get why there needs to be builds for tanks, unless some cdr, ap or a special core build is needed. Just look at your enemies and build armor/magic resist items according to need..