Is it true that the Polish farmers' markets such as Hala Mirowska and Hala Marymoncka
I don't know about those two specific examples but a lot (if not most) Warsaw street stalls selling fruit and veg, who theoretically are supposed to be selling their own produce, buy it at the big wholesale market at Broniszewo (amazing place - worth a visit in the night - good bars and cafes for the workers and drivers). A lot is Polish produce (notice that the street stalls don't usually sell obviously imported things like bananas - they aren't allowed to unless they register for tax etc) but obviously they just buy the cheapest, wherever it's from.
I have a friend with such a stall. Some of the things like potatoes and ogórki are from his own farm but selling vegetables takes a lot of time and hard work, and to grow and pick everything as well would be an impossibility. The real money apparently comes from summer fruit, which also comes from the wholesale market but is almost certainly local.