Now, audits are generally not worth very much (even, perhaps even especially, from a Big Four group like KPMG), but for this type of thing (verifying that a company isn't doing something they promised they would not do) they're about the best we have.
Secondly, the idea in audit is not really about digging into the engineering. So although they will need people who have some idea what DNS is, they don't need experts - this isn't code review. The auditors tend to spend most of their time looking at paperwork and at policy - so e.g. we don't expect auditors to discover a Raspberry Pi configured for packet logging hidden in a patchbay, but we do expect them to find if "Delete logs every morning" is an ambition and it's not anybody's job to actually do that, nor is it anybody's job to check it got done.
"to audit our code and practices annually and publish a public report confirming we're doing what we said we would."
I run an investment fund (hedge fund) and we are completing our required annual audit (not by KPMG). It is quite thorough, they manually check balances in our bank accounts directly with the bank, they verify balances directly off blockchain (it's a crypto fund) and have us prove ownership of keys by signing messages, etc. And they do do a due diligence (lots of doodoo there) that we are not doing scammy things like the equivalent of having a raspberry pi attached to the network. Now this is extremely tough of course, and they are limited in what they can accomplish there, but the thought does cross their mind. All firms are different, but from what we've seen most auditors do decent good jobs most of the time. Their reputation can only be hit so many times before their name is no longer valuable to be an auditor.