Seems like there is a wide gap between a single company organizing a union for their employers and a monopoly on the labor supply. I would be more convinced by your point if this wasn't one of the first tech unions (according to headline).
If the whole union will quit unanimously if they do that, then they have effective monopoly power over labor for the company.
The company will die if they try to bypass the union. In this case they could distort compensation far above what a competitive market would bear, and consumers of Kickstarter eat the cost (e.g. broader society).
It's definitely not a free lunch where workers get paid more and there's no societal cost, as many would like to believe
This is no different from having to accept any other company policies.
A company has a monopoly on its own policies, including whether joining a union is required.
Employees can join a different company. Companies can't magically become a different company. If they have a single union they are forced to work with, that union has monopoly power on labor supply.
Good for union employees, bad for the rest of society