There are actually protections for striking according to the NLRB “depending on the purposes and means of the strike action.”

I believe if it’s an unsanctioned wildcat strike they can fire without it being retaliation, but if it’s a strike backed by the NLRB they have to prove it isn’t retaliation and I don’t think the days gone by an official strike counts here. I could be wrong but that is what I remember about wildcat strike differences

It depends on the kind of strike. Workers that strike over a company’s unfair labor practice are protected from permanent replacements. This is not that tho. While they are far apart, I don’t think they can accuse the companies of refusing to negotiate.

Of course if the union wins the strike then no replacement.