1. Their job easily replaceable by any english speaking, high school educated worker on a planetary level.
2. There is almost no capital expenditure to setup a 'moderation centre', just setup a bunch of computers in an office and you are done. So workers don't have leverage over a company that is unwilling to move.
3. AI will annihilate probably 80% of the offshore call center business in a few years, the workers laid off from call centers, can 1-1 slot into these AI-RL training jobs. Heck AI will also annihilate 80% of those offshore moderation jobs for say facebook. So more competition.
Frankly, the AI companies doesn't even need to move to another country, they can just hire non-unionized workers in those same african countries. When they will do this, depends on when the union asks for too high of a wage.
The most optimistic outcome, is the unions try to work with the AI companies in asking for better conditions, but not neccessarily much better wages. Overplay their hand and they will fail.