This is not an informal discussion though. This is a monthly thread, posted by one of the administrators of the site, with the express purpose of matching job seekers with job providers. The top level post includes everything a normal job posting does, and is phrased in the style of a job posting. It would be hard to argue in a court of law that it is not, unless the OP doesn't have permission from Apple to advertise a job opening.
They will probably send you a link to formally apply on the Apple site once you send the email. I’m guessing that site will have the comp range.
Not a lawyer, but I’m guessing the comp range inclusion needs to be advertised before you formally apply for the position. I doubt the email here is that.
It's less a formal post and more like "Hey I'm on a team hiring, here's what the job entails, email me (or recruiter) if interested!"
Yes it might technically amount to a job posting, but being pedantic and making this forum a hellish place probably isn't a good idea :)