Whatever the excuses, this is just stupid because it is so easy to get right. Don’t invent your own random number generator, use the one from the OS.
Keep in mind that 7-Zip is a fairly old app - it first shipped two years before Windows XP, which also happens to be the first version of Windows that had cryptographic APIs, and in particular, CryptGenRandom.
I mean, yeah, 7z should fix this, but they're still going to be using unauthenticated CBC, which is a much bigger problem, so I'm going to have a hard time getting too worked up about this.