That retort retrocop overlooks a couple of points: again, the point is not that such collaboration is impossible from outside the EU, but rather that it's easier from within. For example, the relative ease of movement within the EU makes travel, and discussion, and active collaboration, much more practical. And vice versa, as it happens. I don't think it's just because of geography that in my University the relative demographics of people in the department are weighted in favour of EU citizens -- and while there are some non-EU citizens around, there are a) fewer and b) the rules surrounding them are a lot more stringent. Presumably, the entire point of leaving the EU would be to extend the tighter rules to all immigrants. But one of my non-EU colleagues has already been obliged to leave because the stringent rules have forced her out, or are soon about to. In the same way, I don't think it's just geography that explains why most conferences hosted here, or that I have attended, that are meant to feature leading experts in the field, are overwhelmingly attended by EU citizens, or people from institutions within the EU. The Open borders of Schengen drive that ease of collaboration, where you can hop over at barely a day's notice to work with someone in the flesh, as compared to the longer-term planning that has to go in to trips to the US, India, Russia, China and other countries.
People still go to and work with people in these places, for sure. But it is harder. The rules are tighter. Collaboration is often more constrained. The set of cooperative rules that apply within the EU make that collaboration much easier, much more fruitful, and much less of an administrative headache.