grammaticality - What exactly is a flat adverb?

I know that a flat adverb is an adverb that has the same form as its related adjective, but does that mean any adverb without the -ly suffix is grammatically correct? For instance, if I said I am “extreme angry,” is that the same as saying I am “extremely angry”? Or is saying that it is “dangerous hot outside” the same as saying it is “dangerously hot outside”? I realize that flat adverbs are less common than they used to be, but that doesn’t change the question: Are flat adverbs always formed from words, namely adjectives, missing the -ly suffix?

Thank you

PS: This is what prompted my question:

“Flat adverbs used to be much more common than they are today. For example, in Robinson Crusoe, Daniel Defoe writes of weather that is "violent hot." Samuel Pepys wrote in his famous diary of being "horrid angry." But most of these adverbs have long since been abandoned.”

Bolded words are my emphasis.