they didn't "hack" the machine using heat!
they gained control of both machines ahead of time, and THEN used heat (etc) to exfil data.
they didn't gain control of an otherwise stock computer using heat over air gap. stop saying "hack".
I'm afraid you don't understand the meaning of the word "hack" in this context. It does not always mean "gain control/privileges on a computer system in excess of your authorization". In this context, it means "defeat a method used to guarantee a particular security property".
Property: No control/data flow shall pass from the outside world into this computer
Method: Air-gapping that computer
Hack: Defeating that property and passing data between the machines
Let me give you another example.
Property: Computers in different classrooms shall not be able to talk directly to each other despite being on the same physical network
Method: Assign each classroom a VLAN and enforce that at the switch
Hack: By Double tagging certain ethernet frames you can defeat the property.
Now you are going to sperg because no one gained control of anything (even the switch). But of course it's still a hack -- you have shown that the switch + VLAN configuration is not capable (in its current configuration) of providing that guaranteed property of non-communciation between VLANs. In some sense this is actually a more elegant hack than taking control of the switch for obvious reasons.
TL;DR Version: "Hack" means to gain advantage or defeat a security property. Sometimes that involves traditional exploits/privilege escalation, other times it involves other methods.