FYI:- Once Matzo is baked, it can never become Chometz.”
Those that hold by non-gebrokts do not agree with that statement. I personally do eat gebrokts, but those who don’t, have a chashash that a speck of flour did not get kneaded into the matzah dough thoroughly, and therefore never actually baked with the rest of the dough (unless you agree that any flour that is exposed to a high temperature is baked). So when that speck of flour comes into contact with a new liquid (i.e. soup and knaidlech), the flour that previously was not mixed with liquid has now become mixed with a liquid and can become chometz in 18 minutes. At least that is my understanding of the issue. Those who eat gebrokts believe this possibility to be less than nil, and that once baked, matzah is matzah and as you said, cannot become chometz.
I have another question that has always bothered me. What is this “18 minute matzos” business. ISn’t all matzah under 18 minutes from start to finish, or it becomes chometz?