I have already explained that in a few posts furtherer after the one you quoted. Her Northern "colleague" was making exactly the same visit, on exactly the same protocollary level, to President Obama. But she knew who she is, what she represent. She looked every inch a Queen, wearing hat, gloves, tasteful diamond-and-pearls bijoux. There was no any misunderstanding. Doña Letizia wore just a summerdress and that was it. She could easily have been the tall bearded guy's assistant instead of Her Majesty The Queen of Spain. Thát is what I meant with lack of decorum and "come on... show a bit more stature!".
Máxima at the White House:
see picture
Letizia at the White House:
see picture
Who looks more like a royal lady indeed, dressed to the nines, to honour the importance of the occasion and the host? But never mind, I know taste is in the eye of the beholder and I think Letizia should (and can) outshine the whole bunch. Even with Zara clothes.