Not sure I'd call that author mainstream. A couple chapters later he starts blathering about "the device" and talking about gaskets and hydraulic force and water flow. It's badly formatted, there are no section breaks, he keeps referring to figures and diagrams and photos that aren't there...This dude's a crank.

My understanding is the mainstream theory is that the portcullises were resting on wood or stone supports that were set into slots in the floor during construction, fitting and the funeral service, and then when it was time to seal the pyramid the portcullises were hoist off of their supports with ropes suspended from logs across the ceiling, the supports lifted free and removed, and then the ropes eased to lower the portcullis. You wouldn't want to let them slam shut, granite is very hard but very brittle.

If you can hoist the stone off of its supports to gently lower it, you can hoist it open again. Whether they ever did isn't knowable.

Compare that to the design of the portcullises in the Bent pyramid, built by Khufu's father Sneferu. They slide diagonally into place into deep recesses in the wall and floor rendering levering attacks entirely impossible. It's practically impossible to open once closed, your only way in is to tunnel through. Which is what has happened to the one that was closed. The other was never closed and remains propped open with an ancient log to this day. I'm wondering if it closed by accident. Notably, Sneferu's later Red pyramid has no portcullis system at all, and Khufu's portcullises were made of granite but of a much less elegant design.