The paths to the files are ‘absolute’ in the sense that they refer to the position of the files on the drive. Moving files to a different volume will definitely break the paths.
The paths are not as ‘absolute’ as absolute hyperlinks in a Web site. OS X can still find files so long as they remain on the same volume.
See DT Pro’s Scripts > Path & URL > Update Paths. This can help fix broken paths, e.g. when you transfer everything to a different computer or move stuff around on different volumes. But it is still a bit tricky.
Although my databases are primarily self-contained, I still have some external files (Word .doc files and some very large Indexed PDFs, images and QT media). I put those external files in a folder named “Fodder for DEVONthink Pro” to make it a simpler matter to move to another computer.