Hence my "for the most part" and frankly there still isn't that much separation between Arnold and Jake Gyllenhall. You're just more familiar with Arnold's shitty work. Watch Prince of Persia or Accidental Love. There's 1000 great actors for every role, easily, but there is only one Arnold. No one else could have played Conan or The Terminator as well, no one.
I guess I don’t understand what you mean by “for the most part”. There certainly is “acting ability”. Those who can do and those who can’t don’t.
You can make this distinction for any elite endeavor, then. The difference between a great college football player and an elite level NFL player is splitting hairs in terms of athletic ability and effort. But a 5’4 160 man isn’t going to play as an offensive tackle.
In the 2 movies you mentioned Arnold played roles that called for his physical appearance. I will argue that he is a worse actor today than he was in terminator. Gyllenhall is a better performer today than he was in Prince of Persia, but that was a movie based on a good game, no actor was saving that.