We don't know 'exactly' what the deal was with the regulator.
The assumption at one point was "new deliveries had to have the fix"
Another view might be "new deliveries can only happen once a fix is available" - which is slightly different
I'm sure there are others.
All we can do is speculate, and to add to that speculation, the regulator is probably still procedurally in a world where cars went back to dealers for updates and so maybe, wild speculation on my part, there is a 3 month window to have cars fixed once the fix has been approved for use, and BAU can otherwise resume. Tesla had the fix, regulator said 'great... carry on and confirm by Sept that the cars are all done" and away we go.