This one surprised me:

  Helvetica
  Win: 7.34%  Mac: 100%

I know Microsoft prefers Arial over Helvetica, so I guess I shouldn't be surprised. But I didn't expect macOS to adopt it so widely.

  Arial
  Win: 99.84%  Mac: 98.74%

Then again, the Mac was designed from day one for this sort of thing.

I believe Helvetica has shipped with every Macintosh since the very first one.

Not quite. The original sans font on the Mac was Geneva. It was only with the advent of the Laserwriter that Helvetica became a standard font on the Mac.

If you're really targeting either, you're probably better off with the fallback sans-serif which is often one or the other based on the platform (historically, not 100% sure on current browsers). Arial displays slightly better on Windows, and Helvetica likewise with Mac.

It just means it's installed by default on all Macs, but not Windows