It simple means that the historical recollection is false.
"the gully through which the celebrated Light Brigade had earlier charged to its slaughter" - this does mean that they crossed through the gully on the way to losing the battle and getting slaughtered. However, the next line refers to the gully as an "invented site of memory". It is recorded, in the history books, but they never went through the gully. Another example: July 4th is Independence Day in the US. History documents it as such. However, all the signatures, on the Declaration, were not completed until July 7th. July 4th still keeps the honor. I hope this helps, Stephen