I could write that it is 2 to the power of (1/12), but 90% of the worlds population have no idea what it means. It is easier to remember "6 percent" than "2^(1/12)"
Of course you can, but that's not what's implied by the quote.
In addition to microtonality, many instruments (and the human voice) are capable of infinite tuning, and musicians will deviate from the "correct" note (as played by an even-tempered instrument) based on chord structure, voice leading, etc.
That's not what "tuning" means. Yes, fretless string instruments, the human voice, instruments like the trombone are all capable of being played with arbitrary or semi-arbitrary pitch. But they are still either tuned in some way, or never tuned at all. Performance is not tuning.
Aside from that not being what was said (the parent’s pull-quote is wrong in multiple ways), it is in fact not possible to tune a piano to any arbitrary set of 88 pitches without changing the design so dramatically and fundamentally that it would no longer be a piano. The triangular shape of a grand determined by the strings lengths, the placement of the pins, the use of double/triple strings in some frequency ranges, and design of the sound board, all permit only a very limited range of frequencies for each key on the keyboard. You very obviously cannot take an existing piano and tune it to 88 keys of A0, nor to 88 keys of C9.
Since an "arbitrary piano" is tuned by turning screws, like simpler string instruments, the only pitches that it cannot be tuned to are those that fall outside the range of each string. Departing from the usual inharmonically tempered 12-EDO tuning would just sound strange, possibly outside some piano tuner's professional comfort zone.
And that isn't a nuanced opinion; a common one among rigid materialists. Its not the best piano for a stage, but I am sure it could fool your ears in a recording. Morpheus would think it was a piano too. "What is real? ..."
You were responding to a comment about how difficult it is to achieve an arbitrary tuning in a piano by bringing up a software piano. It’s a non-sequitur.
Alice: “I have this piano that I need to tune, but tuning pianos is very difficult and I don’t know how”