To promote my own tool, https://github.com/bradfitz/shotizam lets you drill down into why Go binaries are large without having to make up terms like "dark bytes".
As explained in OP, the sum of sizes advertised in the symtable and ELF section headers does not add up to the final binary size. The shotizam tool is thus blind to that difference.
Brad is a good person, it's the code we're talking about here.