I think you’re a little overly strict in your definition of “single source of truth” - if you have a “system” instead of “one piece of technology” then I don’t see a problem saying that that is the source of truth, but yes, you have to make sure everything is correct behind the “public api”. Maybe a bad example, but going further you could say “oh because of the normalization in the database table X is not the single source of truth anymore because I have to use JOIN”.
That said, we all know how encapsulation and access works in the long term so I can 100% understand the way you see and described it :P
It is mostly about the different workflows. One piece in Netbox where changes are applied immediately and not versioned. One piece in Git where changes can be reviewed and versioned. If you modify only one, no problem but if you need to do a change spanning across both of them, you are running into trouble.