I think it's a waste of time and money on their part. The problem is we don't have a Loser Pays All law in this country and Home Depot will have to pay big bucks to their legal team for defense.
Here is the NLRB claim from the link in the OP:
According to the NLRB, the actions taken by Home Depot were a violation of the National Labor Relations Act, which protects the rights of workers to join to improve their working conditions under what’s known as “protected concerted activity”.
Jennifer Hadsall, the NLRB regional director who issued the complaint, said the agency “protects employees’ rights to raise these issues with the goal of improving their working conditions” and that “it is this important right we seek to protect in this case”.
What judge or jury in the world would agree that wearing shirts with radical messages on them would improve working conditions? It's such a stupid claim before the case is even heard. If anything, it could create a conflict with other workers. If I were working at Home Depot, and they allowed lowlifes to wear BLM shirts, then I would claim the right to wear a MAGA shirt or red cap.