I do not think this is true, and I will state my reasons.
Potatoes in Europe did not take off as a base crop until the early 19th century, at a time when armies marching across the continent was rare and noblemen stealing crops was even rarer, due to the development of the state and its monopoly on violence. Contrary to many popular beliefs, noblemen rarely stole crops - they demanded payment for land rent, regardless of the condition of the harvest and might send in collectors if the tenant could or would not pay. But they did not ride around stealing crops.
Potatoes were not left in the ground - if they were, they would rot as the ground is wet. They were stored in a stuka (unfortunately I have no idea how to translate that), which is a temporary earthen cellar, often built directly in the field and the harvest collected there.
See here for an illustration of a stuka. Basically, you dug a shallow pit in a well-drained and dry piece of ground (often the middle of the field harvested) put hay in it, put your potatoes in a pile on the hay, covered the pile with more hay and covered the it with dry earth, leaving some of the ground hay sticking out for ventilation.
These piles, while still out in the fields, would be easy to recognise and plunder - almost as easy as a granary or a grain storage in a barn. I can't see a real advantage against plunder between a stuka and a granary.
Potatoes gained massive popularity because it is a very easy crop to grow.
It requires a minimum of work until harvest.
It requires very little work to actually harvest compared to grain.
It requires very little work to process after harvest compared to grain.
It will grow well in marginal land that cannot sustain a grain harvest.
It is very productive compared to hardier grains - a normal harvest of rye in early 19th century Sweden would be twice the sewn amount, while potatoes could be 3-5 times the planted amount.
To give a comparison - grain need to be scythed down, cut (the stem separated from the grains), treshed and chaffed (extremely laborous until the invention of mechanic treshers) and then milled. Even after you have flour, you still need to mix, rise, knead and bake it into bread. Potatoes? Bring a bucket, rinse, boil, eat.
Note that wheat is also very productive, but many places in Europe could not grow that much wheat until hardier kinds were introduced later. Potatoes were an excellent option for places where wheat could not be grown.
Absence of evidence is not evidence of absence, but I am inclined to believe that this is not true - potatoes became popular because they were an easy crop to grow, required less labour to could be grown in marginal land and was very productive - and you could make vodka from it.
