AAC cannot go below 32

[b0red]

The rest of the non-proprietary format world moved to using Opus.

"AAC starts sucking with lower bitrates"
which is exactly the point, to fit 8mb videos on Discord

Discord uses Opus as it's default audio codec (specifically a sub 64kbps subset from memory), ditto for WebM videos (which hybrid can also make). It performs better at low bitrates than AAC.
Not specifically sure if every version of discord plays videos the same but all the modern web browsers and OS's discord runs on have it either built-in or support it.