In short: Yes and no. Yes, Micro Snitch works on these computers just like it does on any other supported Mac. And no, Micro Snitch does not show microphone activity when your Mac is listening for “Hey Siri” – which is all the time.
To be precise, Micro Snitch will not show microphone activity even when you talk to Siri, regardless of how you activated it (by saying “Hey Siri”, by clicking the button in the menu bar, by using a keyboard shortcut, …). The reason for this is that nothing in macOS actually accesses the microphone when you use Siri on such a Mac. Instead, the T2 Security Chip handles all audio input without actually passing any audio data to macOS.
Of course, Micro Snitch works as expected when any other application uses the microphone.