The whole paragraph from page 9 of the manual:
"The effective mass of the arm horizontally is equal to the sum of its component parts. (It does not pivot) It needs to be as light as possible for low mass, however, making the arm too light sacrifices rigidity. By decoupling the counterweight system horizontally, but not vertically, the mass of the counterweight is not seen by the cartridge above a certain frequency and is lowered. This allows the use of heavier (more rigid) components in the tonearm design without increasing the effective mass."
He is, indeed, talking about the rigidity of the tonearm parts. Throwing weight at the I-beam near the spindle pivot does not increase the arms rigidity.
"The effective mass of the arm horizontally is equal to the sum of its component parts. (It does not pivot) It needs to be as light as possible for low mass, however, making the arm too light sacrifices rigidity. By decoupling the counterweight system horizontally, but not vertically, the mass of the counterweight is not seen by the cartridge above a certain frequency and is lowered. This allows the use of heavier (more rigid) components in the tonearm design without increasing the effective mass."
He is, indeed, talking about the rigidity of the tonearm parts. Throwing weight at the I-beam near the spindle pivot does not increase the arms rigidity.