Uhm, I don't know the academic terminology but I believe this sentence refers to the constant revision of history as "we", in the general sense, know and understand it.
For instance the novel refers to the Victorian age. How we understand the Victorian age in 2003 is probably vastly different than it was understood in say, the 1950's.
The more the years past, the more voices contribute to the discourse, the more it changes. The Dead White Guy version of history was once considered perfectly reasonable and acceptable. Not so much anymore.
I liked this particular sentence. thought it summed up the concept quite nicely.
An important element of this work is that it is firmly set in the world after 9-11. In Gibson’s view, 9-11 was the end of history; after it we are without a history, careening toward an unknown future without the benefit of a past---our lives before 9-11 are now irrelevant.
quote: And from where they are, the past behind us will look nothing at all like the past we imagine behind us now.
My understanding is that sentence refers to what WG call as "the infinite plasticity of this form of speculative fiction called history". 9-11 is probably just a reference point.
Refrencing the Victorians: they were the Brittish Empire, saw themselves as having the right to go live in foreign lands and "save" the natives with Protestant christianity. In 1903, that was their storyline. In 2003, we see the 19th Century as more of a whole, and we also see how 1903 turned into 1904 ... etc. We look at that attitude amongst the Vics and marvel at it's sense of arogance and condescesion. We also know much more than the average early 20th century person about the human rights abuses that went along with what is now a dirty word - "colonialism".
It's like that saying, "you can't see the forest for the trees". The understanding people will have of our present (2003) in a hundred years will be infused by stuff like declassified government information, among other things, so they'll have a whole other, perhaps more simplistic, understanding of our bewildering now.