I'm inclined to believe it. If someone managed to prove Apple's lying about it, there would be serious reputational (and other) risks to their business. I also can't imagine how they would benefit from such a fabrication.
That said, I still use "Nanoblock" webcam covers and monitor for when either the camera or microphone are activated.
It's clear Apple define "Hardware" as "Not using the main CPU". They've pretty much admitted it's firmware based, otherwise the T2 chip simply wouldn't be involved to be mentioned.
It is implemented in dedicated small CPLD that cannot be flashed by any software means. My understanding of relation to T2/SEP is that this CPLD serves as a kind of "IO expander" for T2/SEP which also hardwires logic like this.
The T2 chip is mentioned in the quoted passage as an indicator of the architecture version, not necessarily an indicator that the T2 chip is directly involved
We have no way of verifying that anything they said in that document is true.