That seems like a weird assertion on the part of whatever you read, because 1) it isn't clear what constitutes a separate, well-defined language (see https://en

That seems like a weird assertion on the part of whatever you read, because 1) it isn't clear what constitutes a separate, well-defined language (see https://en.wikipedia.org/wiki/Dialect_continuum https://en.wikipedia.org/wiki/Dialect_continuum) and 2) mutual intelligibility is a gradient, not a binary. Many speakers of Standard American English have to turn on subtitles to understand what's going on in movies from the UK (like Trainspotting). So maybe we call a great many things English?