Thanks for catching that typo! always good to have another pair of eyes.
Deja vu is a misfiring of neurons in the brain by which things you have not seen before trip the memory trigger and make you feel like you have. First-hand experience with this due to epilepsy- when the wires go cross that's a big side effect- everything before a seizure feels like the strongest deja-vu you've ever experienced.
As far as measuring time as a spatial dimension, yeah it's a bit of a trip. The best way I've been able to imagine it is taking a literal view of the Maori phrase- imagine all your life you've been walking backwards without realizing it. This is like that, only instead of going straight backwards imagine yourself going in an angle that doesn't exist to your perception. The retina is not 2D, the rods and cones are 3D cell structures and even if they weren't there are enough of them to recieve light from enough angles to create three-dimensions. Illusions and forced perspectives actually rely on 3D vision, tricking your eye into thinking there is increased or decreased depth. a two-dimensional being would theoretically see a two dimensional object, just as we see a table. They would be able to extrapolate the third dimension by virtue of the existence of say... the two dimensional shadow of three-dimensional things. We can see three-dimensional shadows of the fourth dimension, as Sagan pointed out. There's a 4D sandbox that aproximates a representation of 4D objects interacting with the 3D world digitally.