I believe the files were imported and renamed in their native proprietary format. The question of DNG for Seth is only to export a DNG after his development etc. to a Job folder (not necessarily part of the LR Catalog) and then only as part of his deep backup process - the part referred to by Michael as the 'belt, suspenders & a piece of string'.
Pretty sure Seth does indeed import native files and only uses DNG on an export to adjusted files with metadata...the primary reason is that any backup software that will catch file modification date/time and copy those updated files will have to copy the entire DNG file each and every time that file is touched...and a raw file may be 10-15MBs in size when saved as DNG. However if Lightroom is working on native raws instead of DNGs and updates to those files get written into the .xmp text files which is tiny (usually under 100K). So, you can accomplish a lot more small file backups than full file DNG backups...
I also do not convert to DNG upon import for Lightroom and only convert to DNG when the raw image processing and metadata is essentially done. Then I'll export the DNG for further use...