The rumor comes from hardware leaker Mauri QHD (via NotebookCheck) who wrote that AMD was working on two versions of the chip—one with integrated RAM and one without. The prototype is "almost ready", apparently.
The Tame Apple Press is furious and claims that AMD is only releasing a spoiler chip to match Apple’s superior chip technology.
However AMD has explored the possibility of an ARM-based chip as early as 2014 when it was creating a custom 64-bit ARMv8 CPU core codenamed K12. And in 2016, it announced the K12 Core. Then AMD lead CPU architect Jim Keller led the project, but the SoC was shelved.
In May of this year, however, leaker Komachi Ensaka created an AMD roadmap that included a "K12 FFX" with a vague release date of between 2017 – 2022. Either way this has been part of AMD’s plans for a while and it was probably waiting for Microsoft to come up with Windows for ARM chips.