I guess it would be safer to say that the while Panglossians (not what they call themselves) may not think human performance itself is normative, they do think that human performance is usually rational. I've edited the OP with this change.

But, to illustrate the difference between the Meliorists and the Panglossians, and why I side with the Meliorists, let me quote from the Panglossians themselves (Todd & Gigerenzer 2012):

We use the term logical rationality for theories that evaluate behavior against the laws of logic or probability rather than success in the world... Logical rationality is determined a priori... instead of by testing behavior in natural environments... [In contrast,] the study of ecological rationality investigates the fit between [the structure of task environments and the computational capabilities of the actor].

But this seems to miss the point made by Baron since the 1980s about the difference between normative, descriptive, and prescriptive rationality. The Meliorists never said that the way to achieve success in the world was to explicitly use Bayes' Theorem 100 times a day. What they said was that human behavior falls short of normative model (from logic, probability theory, and rational choice theory) in measurable ways, and that (just as the Panglossians maintain) we can look at the structure of human task environments and the way the human brain works in order to offer prescriptions for how people can perform more optimally, e.g. by changing the environment (Thaler & Sunstein 2009 or by training the human with a new cognitive heuristic like "think of the opposite" (Larrick 2004).

So do the Meliorists and Panglossians actually disagree, or are they merely talking past each other? Starting on page 10 of Rationality and the Reflective Mind, Stanovich argues that the two groups substantively disagree. I'll let you read that here. Another good overview of the substantive disagreements is Vranas (2000), which reviews the debate over normative rationality between Gigerenzer (a Panglossian) and Kahneman & Tversky (both Meliorists).

I do find it somewhat hard to believe that anyone would willingly label themselves "Panglossian."