Yes, and the only thing that has changed since is the introduction of the Smart and LTR visas. They generally don't care as long as you're not taking a Thai job .... except for when they do.
No work permitted means literally that - you cannot legally work on a retirement visa. As others have said, though, keep it on the down low and you will likely never have a problem, even if it is illegal.
If you've not read it before, this interview with the CM Provincial Employment Office goes through the requirements and more importantly, what they care about.
Business people can enter on a Non-Imm B and conduct business - attend meetings etc, but they can't work. Op is living here and working in Thailand - an appropriate visa and WP is required. However, in practice, we know what local authorities care and don't care about.
Colonel, the agent directly across from Immigration Airport can sort the residency certificate out for 1k. As for the DLT, no choice but to do that in person.