Acorn Computers started off much earlier (I owned an Acorn Atom when it was released) which begat the Electron, then the BBC Micro and then the Archimedes.

At that time ARM was just an architecture owned by Acorn. They created it with VSLI technology (Acorn’s Silicon partner) and used the first RISC chip in the BBC Micro before then pivoting it to the Archimedes.

Whilst Acorn itself was initially purchased by Olivetti, who eventually sold what remained years later to Morgan Stanley.

The ARM division was spun off as “Advanced RISC Machines” in a deal with both Apple, and VSLI Technology after Olivetti came onto the scene.

It is this company that we now know as Arm Holdings.

So it’s not entirely accurate to claim “they had a full computer system” as that was Acorn Computers, PLC.

Some of the other details you have are wrong too, to the point your comment is really quite misleading.

Anyone wanting an accurate version should check wikipedia: https://en.m.wikipedia.org/wiki/Acorn_Computers

(To be blunt the above comment is like a very bad LLM summary of the Acorn article).