I believe this is absolute magnitude, and specifically, absolute magnitude as defined for comets [1]. At Saturn's distance, definitely not a naked-eye object in any case.
Not in this case: the +13 figure is a predicted *apparent* magnitude. The absolute magnitude 'H' as your link defines is the 'H' field in the OP link, which is +7.8. (Note that's 'H' from your link's section on asteroid magnitudes, not comets or stars -- they're all on different scales).
Here's the documentation for the fields in OP's data table:
"H Absolute visual magnitude. A table converting H to a diameter is available."
Ah, thanks. So its H_asteroid is currently +7.8, and if/when it develops a coma it should become a lot brighter, but unfortunately even at perihelion still way too faint for a naked eye.
[1] https://en.wikipedia.org/wiki/Absolute_magnitude#Cometary_ma...