What type of Zoysiagrass do you have?

If you are using a propane torch, the ones used for torch downs (roofing) then you will heat the soil up too much and kill lots of the rhizomes. There are lots of types of weeds that wait for fire to sprout. You will be overrun with these.

I would scalp it short in the early spring.

It is Meyers aka Z-52 the most common Zoysia on the market and about the only variety available variety 15 years ago.

Burning is not going to touch thatch. Thatch is dense and firmly packed against the soil. A fire is not going to get hot enough to burn it out. Get a Power Rake to remove it.

No not at all, burning dormant Bermuda and Zoysia can do quite a bit of good, mostly fertilize it.

Zoysia and Bemuda reproduce by rhizomes (underground) and stolons (above ground) runners. Grass fires are no where near hot enough to burn it. All it does is clear out the dead dormant grass, let the sun shine in, and the ashes renew the soil with rich nutrients.

Look along any highway in the south where there has been a grass fire about a week or two after. It will be bright green and lush assuming some rain and warm weather...

But to the question at hand of burning Zoysia to get rid of heavy thatch buildup it will not work because the fire is not hot enough to burn the thatch off. I twill just pass right over it as it acts like a fire wet blanket on top of the ground.

Now you could lift and loosen it up and would burn if left to dry out. But at that point you just as well rake it up and avoid the risk of burning.

Lesson learned... Always keep a hose ready to put fire out.