I don't believe this. Have you got a cite for this?
Here's an excerpt from the opinion...
"Second, the Union argues that workers do not forfeit the NLRA’s
protections simply by commencing a work stoppage when the loss of perishable products is foreseeable, but this case involves much more than that. Given the lifespan of wet concrete, Glacier could not batch it until a truck was ready to take it. By reporting for duty and pretending as if they would deliver the concrete, the drivers prompted the creation of the perishable product. Then, they waited to walk off the job until the concrete was mixed and poured in the trucks. In so doing, they not only destroyed the concrete but also put Glacier’s trucks in harm’s way. "