Here's a video of a lithium battery fire in a cargo hold, as you can see it causes an explosion and swiftly a fire that burns the entire compartment. If you imagine every checked in bag containing a laptop, you'd get a similar effect if you rig one to catch fire.
Note that the video claims it's 5000 Li+ batteries. It's hard to tell from the video, but it looks like it was ~4 boxes filled with only the batteries. Even a large pile of laptops (no bags) would have a hard time making the explosion[1] in the video. If you separated each battery+laptop into separate bags, the density of Li is so low a single battery burning/exploding may not even reach the other batteries.
[1] obviously a fire is still possible, even with a single battery
Yes I guess it would only contain a few hundred laptops if they force all into the hold. I imagine a fire is quite possible though from a rigged laptop (or just an accident) and impossible to put out, unlike a fire in the cabin.
https://www.youtube.com/watch?v=kd0EzQaEiZQ