The inertia of the ship is based on its thrust, not the direction buttons. The game works the same in arcade and home versions (with digital d pads/sticks). The left/right buttons and a 2 way joystick are identical in function from a technical perspective (sticks are digital, taps and holds are the same on sticks and buttons). They're also in my expectations (and seemingly the author's).

Yes, a spinner would be functionally different (spinning is different from holding), but the question is really about the design choice from the beginning.