One of the key features of a portcullis is because it drops into the closed position under gravity, it's very fast to close. Opening it is slow and involves a winch.
That's in contrast to another iconic medieval castle feature, the drawbridge, which drops into the open position, and closing it involves a winch.
And in contrast to simple doors, which are reasonably fast to both open and close, with no winches required, and which are widely used to this day.
Presumably the article's authors then asked themselves under what circumstances a castle would need something very fast to close - faster than a door or drawbridge - and concluded it was for surprise attacks.
That's in contrast to another iconic medieval castle feature, the drawbridge, which drops into the open position, and closing it involves a winch.
And in contrast to simple doors, which are reasonably fast to both open and close, with no winches required, and which are widely used to this day.
Presumably the article's authors then asked themselves under what circumstances a castle would need something very fast to close - faster than a door or drawbridge - and concluded it was for surprise attacks.