I think he actually means rational. He advocates to make design decisions after reading academic papers on the subject.
Whether or not that's optimal is a separate issue.
The rational part is reading the papers; the optimal part is doing what the papers say.
However, reading papers is not limited to design. It is part of general rationality (virtue of scholarship).
Of course, if you search for papers on cognitive biases in web design, then they would tell you about rational design.
So I'm working for a friend's company at the moment (friend is a small business owner who designs websites and a bit of an entrepreneur) anyway, I've persuaded him that we should research the empirical literature on what makes websites effective (which we've done a lot of now) and to advertise ourselves as being special by reason of doing this (which we're only just starting to do).
One thing that I found absolutely remarkable is how unfilled this space tends to be. Like a lot of things in the broad area of empirical aesthetics it seems like there are a lot of potentially useful results (c.f.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3485842/ ), but they're simply not being applied- either as points of real practice or of marketing differentiation.
A fascinating gap.