Here are the sentences you refer to.

"Like one of Field Marshal Haig's family whiskies, Max Hastings is a dram that steadily improves with age.
His own trenchant views on war, and caustic opinions of the commanders who ran them, tended to obtrude too obviously in his early works, suggesting that if only he had been present at key military conferences costly errors would have been avoided.
However, Hastings's recent massive volumes on his specialist subject, the Second World War, have shown why his position as Britain's leading military historian is now unassailable."

None of it refers to his work on WW1.
Has has done no previous work on WW1, so your claim is rubbished.