Why should be SQL keywords, like CREATE TABLE and UPDATE, must be uppercase in WordPress?

Generally it does not (although it is probably a good thing to do and hopefully it is part of the coding standard.

That specific quote refers to the API which lets you add or change columns in the DB. For that to work wordpress needs to compare what the definition of the table in the DB to what the plugin asks it to create/change if needed, and without looking at the code, being specific about the case that is used just makes the “compare” code easier to write.