"ARM stands for Advanced RISC Machines, the name given to the company when it was spun out of Acorn Computers back in the day. It was abbreviated to ARM when the firm went public in 1998."
It was always abbreviated to ARM, ever since the 1980s. The whole reason for "Advanced RISC Machines" was to match the acronym they already had.
This is the second time I've seen someone make this claim online and I wonder where it comes from. IBM's name is still officially "International Business Machines Corporation (IBM Corp.)" according to this legal statement:
https://www.ibm.com/privacy/us/en/.
As for how it's referred to in practice, I've always heard it called IBM for as long as I can remember (at least back to the 70's) and I think that's been the case for much longer.
They were Acorn at that point. That's why the CPU was called Acorn RISC Machine (singular). There was never any company called "Acorn RISC Machines".