> What I took from that is tech workers have an incredible choice in where they can go work because their skills are highly in-demand. So why unionize?

These things are not diametrically opposed.

A union doesn't imply anything other than collective bargaining. Maybe having a union would prevent things like Disney firing a bunch of full time employees after they retrain contractor replacements. This type of nonsense is driving down wages for the entire middle class of workers.