OP is absolutely right about the fact that "true" full screen is not working correctly in DCS. There was a long time when you could fix this by yourself by pressing Alt+Enter. Then something happened and true full screen worked without Alt+Enter, if you had full screen enabled from DCS options screen. But just like OP wrote, this changed again some time ago and all we get now is borderless full screen. This is a bug and should be addressed by ED.
There has always been a very simply way to verify, whether you are running a borderless or "true" full screen mode. When you are playing DCS or any other Windows game, change the system volume up or down. If the Windows volume bar pops up in the upper left corner of the screen showing the current volume level, you can be 100% sure that you are running the game (only) in borderless full screen.