Create an advancedsettings.xml file, and add .flac: Click!
There's no need to add extensions for flac files, it's one of the defaults. Have you defined the SMB share as a source and scraped the media into the Library? (Music is a Library view). Flac files are similar to mp3 and contain tags that are scraped.
Check that your downloaded Flac files have also been tagged sufficiently/correctly.
I downloaded one of the files from site you mentioned and Flac file had NO tag information.
(I use Mp3Tag on Win PC to check if file(s) have tags)
Read important info on Tagging that is required for music in your Kodi library?
Now to scan EVERYTHING, anyone got a good tutorial?
What does "everything" mean? Remember, not all files can hold music tags. That's what MB Picard adds.
You mean scan your prepared music files into kodi that have now been correctly updated and adequately tagged using musicbrainz.
There is no "Everything" mode in Picard, and I would caution against trying to stuff lots of files into the "to be scanned" pane. Your files will not always exactly match; requiring you to drag and drop things, which is easiest when there's only the files for a single album there. It will take time to go through and match/cleanup/correct all your albums. However, this is only a task you'll ever need to do once. You can train your children to do it ($free slave labour) although they often lack the obtuse knowledge of your music collection to tag everything right 
