While access to economies of scale does create efficiencies, it is not a competitive advantage per se.
If economies of scale are accessible at 10% of market share, that creates room for ten firms with the same economies of scale, which reduces your ability to use economies of scale as a competitive advantage.
In practice, this means that access to economies of scale in that particular market is not a defensible type of efficiency (or, at the very least, it’s not fully defensible).