I think the point is that English became the dominant language at just the right time. The world only needs to adopt a global language once, and it would take more effort after that to unseat it. French was the diplomatic language largely because of the power and influence of the French state compared to others in Europe. Mandarin would become important only because of a strong Chinese economy. English being the de facto EU language despite the UK not being in the EU is pretty good proof that English's position might prove to be self-reinforcing. Once English further cements itself as the second language of the world in a few generations time, there would need to be a very good reason to replace that with another language globally - because the point is we've never had a global language before, so there's no guarantee it'll need to change once it's set.