Not exactly. It's illegal to fire people for engaging in protected concerted activity to improve their working conditions. Unionization is one example of something that could be protected concerted activity, but not all tactics for forming unions would qualify as protected concerted activity, nor is protected concerted activity in any way limited to forming unions. (There are some explicit protections for unionizing, but this is the general principle.)

For example, using your employer's Slack to discuss improving things like healthcare, overtime, wages, etc. would be a textbook example of something that would be protected. You need at least two employees (non-managers) leading the discussion though to qualify as concerted, one person complaining is just a malcontent and doesn't qualify for any protections under the law.

Yep. As weak as union laws are in the US, firing workers for attempting to organize is a surefire way to run afoul of the Wagner Act.

As per the article:

> Outsourcing jobs abroad for the purpose of punishing or discouraging union organizing, as opposed to a valid business reason, would violate the National Labor Relations Act.

IANAL but I suspect their only avenue for recourse would be private arbitration as chosen by the employer. This is pretty standard language in modern employment contracts and from what I understand very few things are capable of invalidating it. (unpaid wages is one example I think can void such a clause)