sounds like the place I used to work for. They wanted the tires filled FULL of fluid. On the L2501/L3301/L3901, 1 gal antifreeze and the rest water, to the brim. I tried to tell them that wasn't smart but the boss didn't listen very well. He had his own way of doing things.
windshield washer fluid is flammable. And nasty stuff if you or an animal drinks any of it, or gets it inside somehow. In humans, the methanol causes blindness and with enough of it, death. Also burns, in the daytime, without a flame. So if you had a leak and you drive into the burn pile, you feel the heat but you don't see any fire, so in a lot of folks' mind, since there's no visible flame, it's not "flammable". Good thing, methanol fire can be put out with water.
better to just use the stuff that is specifically made for tire ballast, and fill it at 75%.