This is just completely wrong. You're peddling bad 20-year-old advice that hasn't ever been completely true, and especially not anymore.
First, firmware updates often patch vulnerabilities that would allow attacks without physical access. Most obviously, vulnerabilities in the networking hardware can allow attackers to take control a machine by sending crafted packets. But other hardware issues can also, e.g., make it possible for untrusted code to perform a privilege escalation of some kind. You want firmware updates to protect yourself.
Second, Macs/iOS devices and PCs—through the Secure Enclave and TPM, respectively—offer really good protection against physical access attacks. They verify the integrity of boot components and are only able to decrypt the system volume if they detect an untampered boot chain. Further, they can lock down direct memory access to protect powered-on systems from bypasses to user authentication. This is why nowadays, if a thief steals your laptop and you had the hard drive encryption with hardware security features on (on Windows, if you set up with a Microsoft Account, this is enabled without any further action by the user), you can feel fairly secure that even a technically savvy thief cannot access your data. Firmware updates can fix vulnerabilities in these security systems.