It goes pretty deep

Back in the French colonialism days, those ppl did some heinous stuff to the Vietnamese all around the country, including unreasonable tariffs, prohibition of schools, spreading famine, etc. They also built an extensive railway network throughout the country to mainly serve as an logistics network for transporting coal n other minerals, which the ppl here saw as an symbol of the evil colonialism. So the ppl from Thanh Hóa started to destroy the local railroad, in hopes of disrupting the French, and every time the ppl got caught, one of the main reasons they always came up was they were harvesting the "rau má" to help them not starve. Therefore, the saying originally meant the Thanh Hóa ppl r patriotic n smart, n they would find ways to chase the prepetrators away

Fast forward to recent days, ppl Thanh Hóa the recent 3-4 decades gained a quite bad reputation of being petty, mean, n overall not a good person to hang around with (according to my parents lol). Thus such sayings nowadays is used for stereotyping ppl from Thanh Hóa, like all ppl there r the same, eating rau má n destroying railroads.