That seriously takes some control over one's own handwriting before it is really possible. Artful lettering has its own learning curve. Plus I'm not sure this is going to have benefits over any other sort of art training.

Additionally, I think this bypasses the real goal of writing - you need writing to be both efficient and legible to others as a base function. This is why hand-written notes to myself often are sloppy and without sentence structure, notes shared with others generally meet the above, and sometimes my writing is pretty and nearly typeface, with the "a" and the "g" written as it appears here. I think folks really need just that middle level of writing. I mostly do the pretty stuff so I can occasionally use it in artwork if I so choose.

I don’t see a reason for signatures to be legible, just identifiable.

Well, sure, for just a normal signature we use daily. Poster was talking about making the signature artful. It doesn't matter if it is legible or not at that point, but it still takes some control to pull off.

But then why cursive, and not any other of the millions forms of art?

Yes. Even in handwriting there are more forms than just D’Nealian or Palmer cursive. Why not Spencerian, Fraktur, or Carolingian?