It's much simpler than this and it is about making money.
Whether unions impact the ultimate success (in terms of ability to build) of a company or not, they certainly shift the share of money that is going towards labor as opposed to owners who want a profit.
This lowers the expected return of company equity which means people will be willing to pay less and you will be able to raise less money while you are trying to scale up. A non-unionized competitor will be able to raise more money and if there are positive returns to scale, outscale & outcompete.