Who claimed that they were "the world's best employer" though? Always look at who proclaims that; it's never the employees themselves. Or if it is, it's a subselection of those that actually had something positive to say about it (e.g. higher management, IT, etc, not the warehouse staff).
It's not about the claim, but about the supposed promise [1]. You can't claim that you aim to be world's best employer and at the same time be union-busting.
