I refer you to the following passage. As a native British English speaker, using 'since long' in this fashion is perfectly normal. You find it quite often in official documentation and legal speak.
The meaning is as explained below, very much like you said, 'since a long time ago'.
Hope this helps.
long since adverb
Definition of long since
1: long ago
promises long since forgotten
2: for a long time
has long since been a devoted friend
Examples of long since in a Sentence
Recent Examples on the Web
The site was likely constructed on the bank of semi-stable river that has long since been reclaimed by the ocean.
— Jason Daley, Smithsonian, "An 8,000-Year-Old Platform in Britain Could Be the Oldest Boat-Building Site Ever Discovered," 27 Aug. 2019
As Warzel points out, Twitter amplified those conspiracies via its trending algorithm, which has long since outlived its usefulness.
— Casey Newton, The Verge, "It’s time to end “trending” on Twitter," 13 Aug. 2019
First Known Use of long since
14th century, in the meaning defined at sense 1