https://wiki.sipeed.com/hardware/en/lichee/RV_Nano/1_intro.h...
I assume they didn't intend to put a mic on the KVM product, but they wanted to make a KVM product, already had this SBC product, which reusing their existing stock of helped keep cost low.
Should they have been more up front about it it? Sure, and it's not great that they had a bunch of security issues in the FW anyway, so not exactly great, but "hidden microphone in a Chinese KVM" lets the mind wander
Far more of an issue would be any kind of keylogger built into the software, which is why it's best to go for devices that support open source software.
Apparently in the 50s when he did his National Service he'd been in the Signals but "not in the regiment that's on his papers", make of that what you will.
I have noticed that with PSK modes and particularly PSK31 you can hear "CQ CQ CQ" as a distinctive pattern much in the same way as it is with CW.
IBM spent a fortune developing ATM keypads that - when correctly mounted - had keys that made the exact same noise no matter how you pressed them or how worn they were.
So I don't doubt that someone suitably clever could extract audio from a room and work out what was being typed.