Heated cauls can work, and from the link in your post...

"Now, an acoustic guitar’s top shouldn’t be dead flat (they are arched, typically with a 4.5m – 6m radius), but this one was severely arched. This one was definitely in need of a debelly job."

As noted above, some belly is normal. I believe people often make the mistake of thinking "flattops" should be flat and go to extreme measures to correct a problem which doesn't exist.