The answers you found on that thread, i.e. spoilers have to be automatically deployed, are correct but incomplete.
Actually, what moves the spoilers is either a motor, or your hand.
The motor will be active in the "up" direction when the main gear is on the ground, airpeed above 80 knots, spoilers' handle in "arm" position or reversers levers up. For that motor to be activated in the "down" direction, it obviously needs to have been turned in the "up" direction first....duh!
That would happen by either advancing thrust lever 1 or 3 (which are linked to the "down" channel of the motor, or after mannually placing the spoilers handle in the down detent; in fact, when you replace the handle manually after landing, even passengers in the first calss compartment can hear that motor turn.
As in the air the motor cannot be activated in the "up" direction (see reasons above), it can't obviously be activated the other way, which is why, in the air, there will me an EICAS caution message with music to tell you when spoilers are deployed with engine thrust levers out of idle, that is on 744, I dunno for the other types.
Cheers.