Is that not true? Certainly you can find a period of time when populations in both England and Saxony spoke mutually intelligible tongues. This confusion seems to arise out of a deliberate misinterpretation of your quote out of bad faith, not any semantic misunderstanding of the past.

If you believe this isn’t true you’re wandering into a territory of objectively demarcated language and dialect classification to little benefit. Pedagogically useful I’m sure, but to what semantic end? Hell I thought this was the type of problem linguistics intended to solve.

No, it is true: that was exactly why I suggested it. My point was that rather than saying something along the lines of "English used to be German" (which makes as much sense and is just as true [false] as saying "German used to be English"), it would have been easy enough to quickly say that they used to be the same language.

(Though, as to your point about England and Saxony: (1) mutual intelligibility is not the same as being the same language (2) Old Saxon isn't the same as Old High German.)

[But, you're right about demarcating language: it gets tricky pretty quickly. E.g., from a certain perspective, English (and German, and French, and Hindi) don't exist in the first place. A bunch of people have some sort of instantiations of formal grammars + lexicons in their brains and the output of these is such that some sort of more or less effective communication takes place and so we say these people 'speak the same language'.]

It's also not even clear that Saxon is actually the origin for English; yes the Anglo-Saxons often called themselves Saxons, but it is likely the ethno-linguistic pool they came from was something closer to Frisian than Saxon. Or some sort of Common West Germanic, from what is now Schleswig-Holstein and Frisia, not Saxony proper.

So yeah there would have been mutual intellegibility, because at that point there was likely high mutual intelligibility between all Western Germanic languages. But like you said that doesn't make them the same language. And like you aluded to, it's kind of a lumpers vs splitters thing.