To be fair, its a poorly written summary that can easily cause anyone who doesn't have a background in this area of engineering to assume it's only talking about internal combustion engines because of the first two to three sentence:
"The vast majority of motors that power our planes, trains, and automobiles are heat engines. They rely on the rapid expansion of gas as it heats up to generate movement. But attempts to shrink them by any significant amount have mostly ended in failure."
This could be interpreted by many as "Heat engines, like those we use to power vehicles, rely on the rapid expansion of gas as it heats to generate movement, and engineers have been unable to shrink them by any significant amount". The summary could have been much clearer by defining a heat engine has any device that converts heat to mechanical work, and stated that internal combustion engines are one type of heat engine, but there are other types as well.
Even when I first read it, I was thinking "How'd they even get an internal combustion engine down to 10^7 cubic micrometers?". It would have been helpful if it described that heat engine as well, since it's obviously not internal combustion.