Yes, those plates are the antenna. The size and geometry of wifi antennas are determined by the targeted wavelength, and in this case by the limit of available space. There usually isn't a ground as with many radio antennas, but perhaps a dipole.

If you soldered to the plates, you wouldn't be adding an antenna so much as altering the geometry of the existing antenna.

Personally, I wouldn't mess with such a microscopic connection; but would rather put the dongle at the end of a short USB cable, then find or make a parabolic dish, and mount the dongle so that the antenna area is at the focus.