One-parting some Commodore 64 utilities for fun and profit

The author of the blog post seems to mean what also often was called "linking" on the C-64: Taking some thing which is supposed to occupy different memory areas and used to consist of multiple files to a single file which makes sure everything's at the right memory locations with a small bit of bootstrap code.

Think BASIC plus supporting machine language code, or code plus assets.

If you take it to the extreme, there was a thing called an IFFL loader (e.g. https://github.com/luigidifraia/iffl-system ) which allows you to put a whole file system with multiple streams into a single C64 file. This had some advantages at friction points like where when the file had to switch location between BBSes and Disk Swappers as you didn't need create disk images first and could simply copy the single part files. Always used to be somewhat exotic though when I still was in the C64 scene - not sure how it's nowadays.