Tsouras's previous works have been on WW2 and the American Civil war, so there is no evidence that he has any grasp of Napoleonic warfare.
'Marching to guns' is marching back to the sound of guns without orders. The myth has arisen that it happened in Napoleonic warfare, due to 1) the idea that Desaix did and 2) that Grouchy should have. While I am no fan of alternative history (because actually no-one can have a complete grasp of all the variables), using something, which is not applicable to the subject just makes the whole alternative collapse.
Desaix had not stopped operations, as some of Boudet's troops were well beyond Rivalta, but operations were difficult, due to the fast flow of the Bormida and Boudet turns back when Napoleon's messenger arrives (see Boudet's report http://www.simmonsgames.com/research/au ... ch.html#s2
). The river runs north-south, so Desaix already had rising smoke east of the Bormida and knowing what Napoleon's expectations were, he queried his orders. Napoleon told Savary, Desaix's AdC, where he wanted Desaix to march to and the map http://www.simmonsgames.com/tools/mapvi ... Frame.html
should make it clear how you march from the south to form troops up around Cascina Grossa.