"The fact that Debian has no compunctions about shipping code in the latter category"
I dispute this assertion. For big packages like X and the kernel, Debian developers sure as hell did. That's why so much effort went into dbs and people took up becoming dpkg developers to extend the source package format, even though dpkg's own source code was formidable (as was the threat of Ian Jackson opining in public on one's enhancements) and it had a reputation for being a bourne from which no Debian developer returnethed.
For packages where either 1) the code itself was really small (some consist of only a single program file), or 2) the delta between upstream and Debian was really small (often only the contents of the debian/ directory), a "monolithic" diff was satisfactory and best practice.
But for packages not fitting in these categories, Debian developers started atomizing patches to be of changeset granularity *at least* ten years ago.
Why? Because it was the preferred way to hack.
Because it was the preferred form for modification of the code.
And that's right when the bell should start ringing. I'm disappointed that you have muffled your clapper. But you're not alone.