I was confused on this since aren't demonstrative adjectives phrases like , "this fast" or "this large". If they are, then demonstrative adjectives not necessarily modifying nouns but adjectives, are certainly called not called a demonstrative pronoun.
A pronoun is called a pronoun because it stands in place of a noun. The preposition pro in Latin means “in place of, on behalf of” (well, it is a bit more versatile, but it fits here). So a proconsul acts on behalf of a consul and a pronomen stands in for a nomen.
Marcus Claudiae obviam factus est et eam [= Claudiam] salvere iussit.
Marcus came across Claudia and greeted her [= Claudia].
Here eam (her) stands in for the noun Claudia. It would be cumbersome and irritating to repeat Claudia all the time.
This is called the substantive use of demonstrative pronouns, but they can also be used adjectively and modify nouns:
Marcus infitiatus est umquam se illam puellam vidisse.
Marcus denied ever having seen that girl.
In that case they are called adjective pronouns or pronomial adjectives. However, in Latin at least, they are still considered demonstrative pronouns.
Expressions like “this fast” or “that large” are an English peculiarity that has no direct equivalent in Latin.