Based on the Quicktime File Format, MP4 is most usually only a container these days. Moving Pictures Experts Group developed the MPEG-4 standard with many parts[1] including a codec called MPEG-4 aka MP4V or just MP4, which was still popular for SD video encoding only a few years ago, though not exclusively, and it is still available to encode video in Apple Quicktime (MPEG-4 Basic, MPEG-4 Advanced), ffmpeg, HandBrake, etc., but I doubt that's what GP meant, at least, I'm not aware of any codec supporting encryption, but I really hardly know anything, so there's that. At any rate, "MP4" alone is ambiguous because it can be either the container and/or the codec.
Let each of them know, please, that I and my compadres apologize for referring to MPEG-4 Part 2 as MP4 to save time, and refer to the container as MPEG-4 Part 14 to avoid ambiguity. Major faux pas there, how embarrassing, mea culpa, and thank you for taking the time to set us straight by letting us know what every single other person does.
I agree with your assessment. The reference to “box” makes me think it was definitely the container. I also don’t see the codec called MP4, it’s usually spelled out MPEG-4.
".mp4" is just a file extension, the source of the colloquial name for the container. But both the container and the codec are named MPEG-4 and both are colloquially called "MP4."
"Box level" is apparently referring to types of cryptography, "S-boxes are non-linear transformations of a few input bits that provide confusion and P-boxes simply shuffle the input bits around to provide diffusion"[1] The basic function of S-Box[2] is to transform 8 bits input data into 8 bits of secret data using a precomputed look-up-table (LUT). A "permutation box (or P-box) is a method of bit-shuffling used to permute or transpose bits across S-boxes inputs, retaining diffusion while transposing."[3]
Thanks, I really had no idea what he was talking about... fucking movie atoms. I've always hated those things, and the idea of encrypting them makes them even more sadistic.
[1] https://en.wikipedia.org/wiki/MPEG-4