It was meant as an insult, implying that Mises - a marginalist - was trying to salvage 19th century liberal economics from the collectivist attacks of the Marxist left and the Nazi right, hence the "neo" moniker being attached.
One of the main promoters of this use was Othmar Spann, a rival of Mises on the University of Vienna faculty. Spann was a prominent proto-Nazi intellectual. In 1924 he added a disparaging chapter on "neoliberalism" to the new edition of his economics textbook.
By the time Mises arrived in Paris in 1938 for the CWL gathering, he had endured a decade and a half of simultaneous disparagement as a "neoliberal" by Nazis and Marxists. It should be no surprise that he was not keen to adopt the label himself. – Phil Magness tweet storm.