"I think that last part I underlined is the key that makes me feel the furnace and central air system are 2 different units that each qualify for $600."

You are the one using "Split" to explain this system. I was pointing out a Central AC isn't also a split system, even though parts are outside and parts are inside. My furnace's duct system was "AC ready" and later we added the AC, but that reflects a Central system. It feeds into the plenum. This is not a Split system. It's two different units, and the AC always has a part that is outside.

"But I'm glad my initial question wasn't as "silly" as I thought."

We can only rely on what other's tell us.