Consider this: For a remote exploit, the attacker must be able to download and run code on your Macintosh that will upgrade the keyboard firmware. If the attacker can download and run code to upgrade the keyboard firmware, they can download and run code to do anything.
So we have two facts: 1. A person with physical access to your computer can install a keylogger by changing the firmware on your keyboard. That person can much easier install a key logger that will work with any keyboard, not just an Apple one, by installing a tiny bit of hardware between the USB cable of the keyboard and the USB port on your computer.
2. A hacker who can convince your computer to download and run software on your computer can do anything. But they can't convince your computer to download and run software on your computer, and if they can, then the keyboard is the last of your worries.
3. A danger is that a hacker with access to your computer could install this on his own keyboard at home, then go to your machine and swap keyboards. They would still need a way to extract the information, so they would have to come back to your machine. And again, there are physical key loggers.
Summary: If Macs are vulnerable to viruses, then we have problems. And there are much worse things that a virus can do than installing a key logger. And if some malicious person has physical access to your Mac, then you have a problem, but this key logger in the Mac firmware is the smallest of your worries.