Many castizos are close enough to pass as white, sure but you could say that about any mixed group.

Regardless, there are tens of millions of full blooded white people there. The Spanish Empire had it’s own version of the Nuremberg laws called the Castas & it survived informally long after independence. Once the initial gender imbalance was corrected post-conquest, Criollo (Spanish) families had every incentive to have legitimate white children to carry on their family name.

On top of that there was also a second massive wave of mostly Italian & German migration in the late 19th century, especially to Argentina, Uruguay & southern Brazil. That’s why you see so many blonde haired blue eyed models from places like São Paulo.