2160p means 2160 vertical pixels. 3840x2160 means exactly 3840x2160 pixels. And when somebody says “4k”, it can mean pretty much anything.
So saying that they all equal is plain wrong and causes even more confusion.
E.g. typical “4k” TVs are UltraHD TVs, i.e. 3840x2160.
4096x2160 is Digital Cinema Initiative’s 4K standard and even though some TVs support that resolution, I think their real aspect ratio actually matches the UltraHD aspect ratio.
If the datasheet says “2160p” then it’s inaccurate and doesn’t tell the real limits.