That first reply (in the quora link) is a bit too dismissive
A lot of the instruction names are the same, but the apparent similarity becomes less once your start using them. Using the same names was probably very convenient given that Acorn's previous computers used the 6502, and it would have helped the programmers who need to transition.
But the machine was quite different to program, and the Arm made more innovations than just being a 6502 with more registers. Every instruction being potentially conditional, and every instruction being able to make use of the barrel shifter, were both quite radical at the time and contributed a lot to the density of code in those machines (although they are inconvenient to out-of-order microarchitectures, so have been dropped from V8)
https://www.quora.com/Is-ARM-RISC-derived-or-inspired-by-the...