There's a lot of misunderstanding about public sector unions and concerted activity.

Essentially most of the private sector has some protections guaranteed by law to engage in union activity including strikes. For instance, private sector workers can strike and the employer cannot terminate their employment because of the strike (there are other legal ways to fuck over striking employees but they are limited somewhat).

If public sector workers strike they have no protections. This means if they walk off the job their employment can be terminated. Trump could just tell them they're fired with no claim to unemployment benefits, COBRA, or severance benefits. They can also be banned from federal employment for life.

Federal strikes are also explicitly illegal and employees or the union would almost certainly be fined. They could also face jail time in theory although that's quite rare.

This doesn't mean they couldn't strike, but it is a lot riskier than a private sector strike.