Sad state of affairs when terrible, ugly vulnerabilities are mundane.

If it was a genuine "backdoor" why would you want use a publicly available key?

Being the only keyholder reduces plausable deniability, so maybe.

The private key is on the shipped devices, from my reading.

Agreed.

I'm hypothesizing that you might do this, even with keys intended to be used as a back door, by shipping it on devices, you vastly increase the number of potential suspects for any backdoor abuse.

Continuing this train of thought - a hardcoded password is classic example of a backdoor, and just as "public" as including a private key.

This isn't a backdoor but it is a major vulnerability.

If mundanity is your concern, add an exclamation point to it.

Accuracy helps.