Wouldn't the first apply to moving to the UK too?

Language I can see, but then the German Abitur has a requirement of 2 foreign languages, with potentially 3 (English/French, French/Latin, last one depending on what the school offers, usually Spanish/Italian/I think Russian in NE/sometimes Chinese). Some of these languages are substantially similar, which further widens the field, e.g., a French speaker might transition to Italian easily.

A fair number of Europeans are ridiculously multilingual.

If by multilingual you meaning speaking english in addition to their mother tongue, I agree. If by multilingual you mean speaking fluently another non english language I disagree, only a small minority do. Particularly in a population of technical people (since we are taking about start ups).

In my experience, even educated young urban Germans are mostly not actually fluent (C2) in English. They can just about hold a conversation (probably B2). Many of them speak French only a little worse than that.

Either way, they'd need to learn more if they wanted to have a normal life in an English-speaking country.

If your family can't stay or can't develop their professional careers in the UK then yes, moving somewhere else is easier than a life full of misery and regrets.

We're talking about non-UK EU nationals in Britain so chances are they're moving back to their families, support networks, and native language.

I am a non-UK EU national in Britain, so I'm saying all this first hand.

So you don't speak the native language of the country you might go back to? I'm certain there are emigrees who came to the UK very young whose parents wanted them to assimilate, but somehow I doubt that's the majority. There will also be people, especially Eastern Europeans, who will want to move to a different EU country than their origin, but again this is the same kind of move they made to the UK to begin with, unless English language education has gotten much better since I lived there.

I'm not sure I get your point or what are you trying to explain / show / support.