You’ve misread my comment I think. Unions can coordinate and organise together. So nobody would be leaving their fellow worker to fight by themselves.

Not so. It makes sense to organise in trade unions. The heads of those unions are on the same side most of the time, as it would be in this case, and they can easily coordinate their actions. But in some cases the interests of one trade have no bearing on another, or are even in opposition, in which case it would be somewhere between difficult and impossible to organise a balloted action across the entire union. Thus nullifying the strength of the union and playing right into the capitalist’s hands.