If I put that through ps2pdf and remove compression, I can see:

  stream
  q 0.1 0 0 0.1 0 0 cm
  /R7 gs
  10 w
  0 G
  1500 1000 m
  1500 1276.14 1276.14 1500 1000 1500 c
  723.859 1500 500 1276.14 500 1000 c
  500 723.859 723.859 500 1000 500 c
  1276.14 500 1500 723.859 1500 1000 c 
  S
  Q
  endstream

i.e PDF has no arc command, it was converted to "c" bezier curves.

Yes, but as someone else commented the arc is internally based on bézier curves. How accurate thee circle would be by plotting is probably dependent on the trigonometric table built in to the postscript interpreter, but at least it would be mathematically possible to make an accurate circle where it's actually not even possible to express a perfect circle with bézier curves.