While the stereotypical conquistador costume is a morion and plate corselet, the morion wasn't in style until the 1540's and cotton armor is indeed the only sort that Diaz mentions. There is lots of 16th century artwork which shows the conquistadors wearing a morion and corselet, but that artwork was usually made decades later and by people who weren't there. The adventurers who embarked with Cortes were mostly very poor, and the prices of everything in Cuba were highly inflated, including crossbows, harquebuzes and horses. The counter-expedition led by Narvaez was made up of wealthier men, and perhaps they and the later Spanish reinforcements were equipped with some metal armor.