Decided to check the primary sources on Lomborg on the ice shelf. Specifically the following paper:
Ice shelf history from petrographic and foraminiferal evidence, Northeast Antarctic Peninsula
Statement of the paper’s purpose:
This paper investigates the Holocene history of the northern Larsen Ice Shelf to determine whether the recent retreat pattern is unique or has previously occurred on a millennial time scale.
And it concludes:
All inshore cores show petrographic evidence for ice shelf breakup at some stage in the post-glacial section.
Whereas the present and historical changes in ice shelf extent are taking place over years to decades, during the mid-Holocene the change from a restricted to a diverse IRD assemblage (ice shelf to open water) was slower, i.e. occupying centuries. Ice shelf re-establishment was also quite gradual.
The inferred period of ice shelf absence, i.e. mid-Holocene warmth, ocurred later in the northern Larsen area than at the George VI Ice Shelf west of the Antarctic Peninsula, but at approximately the same time as the retreat of the Amery Ice Shelf in East Antarctica.
This stuff is quite dense, but the plain reading of it is what Lomborg says: the whole thing broke up (which includes the Larsen B shelf) and then reformed at some point within the last six to two thousand years ago.