It’s twofold. That’s half of it. The other half is there’s a piston in the clicker button that pressurizes air against the ink in the cartridge so it’ll do all the usual writes-upside-down-in-space-underwater malarkey. Only without the need for specialized pressurized cartridge.

Having the hold-open on the clip, separated from the pressurizer, probably immensely simplifies the mechanical design. And probably ironically makes it stronger.

(And I carry my pen that way all the time.)