Oats do not have gluten so they cannot rise - like rice and other such grains. I could understand why they would be kitniot but why are they chametz? I know in arvei psachim there is an opinion that even rice is chametz and rice flour can be used for matzah but we do not paskin that way. Is there a separate specific Talmud source for why oats are chametz?
There is no special Talmudic dispensation regarding oats.
The Mishna lists a grain called שבולת שועל as able to become Chametz. Most Rishonim don't identify that grain with what we call oats. Some Rishonim do identify that grain with what we call oats.
Your point is one very strong proof for the former position, according to which indeed oats can't become Chametz, can't be used for Matza, don't require Challah to be separated from them, etc.
The other Rishonim who identify oats as שבולת שועל may have simply been mistaken, maybe referred to a variation of oats that had some gluten, maybe we've mistinterpreted them and they never intended oats, maybe oat plants evolved in the last 1000 years to no longer rise, maybe oats can rise under special conditions, etc. We just can't know for sure.