Discovery also had hologram communication technology that I guess was also a secret?
No, it wasn’t secret but that also wasn’t invented by Discovery. It was invented by Voyager. Flashback. The episode where Tuvok goes back onboard the Excelsior and they start talking about holographic imagers. Those imagers were created specifically to take holographic image. You cannot take a holographic image without the ability to project a hologram. Moreover, Enterprise showed the crew interacting with holographic technology them. So if you want to complain about inconsistency of holograms in canon, you cannot point the finger so easily at Discovery.
If it was just one thing, okay, but there were such numerous inconsistencies, it was like the writers and designers did not care about trek, they were writing a sci-fi show with the trek name slapped on top.
This complaint gets trotted out constantly. It’s tired and old and frankly it’s dead. There are no violations of established canon in Star Trek Discovery, as much as everyone wants to say that it is. The only examples I’ve ever come across from people, and I use the word examples quite wrongly, are the DOTs, Burnham being Spocks sister, Holographic Tech, and the klingons looks.