Perhaps not obviously, the question was how they could limit your LAN to 10 devices. AT&T's static Internet IP address policy has absolutely nothing to do with this.
AT&T stopped providing static IP's for DSL customers 2 years ago. Had that confirmed by AT&T's customer service and an AT&T engineer.
Indeed it is but that's well beyond the desire of many and something that they wouldn't bother to figure out.
Also, it's rather easy to set up your own wireless router. All you have to do is connect it to one of the free ports on AT&T's router. Been there, done that with clients many times.