The pool of kids from English-speaking countries at those schools is not all that large for a city of 800,000 odd inhabitants.
What's so important about them being from English speaking countries? He was talking about English speakers, and there are plenty of mixed couples in big cities in Poland where the common language is English at home. For instance, I know a Polish-Italian couple who use English exclusively at home. Their kids speak English as a first language, not Polish.
The big problem is that quality educational choices are limited and rather expensive.
Not everyone is obsessed with schools offering 40 hours of science a week to the detriment of all else. There are plenty of good schools in Wrocław available for free or for a low price, many of which aren't so-called international schools, but which offer a quality education in Polish. I have a good friend there who teaches science in English in a public school, and the science programme is taken straight from the UK programme. The kids that finish the high school have the opportunity for doing the English A-level in the chosen science subject(s), all funded by the local government.
In his case, it really won't be difficult to find families where English is spoken at home.
because of the lack of good employment opportunities, abysmal savings potential, and social isolation on the part of the non-Polish speaking spouse.
There are plenty of good employment opportunities in cities these days.
Savings potential? I think we can all agree that other places offer high potential, but also offer a considerably worse quality of life. I could probably earn three times my salary in the US, but would I want 2 weeks holiday? No.
Social isolation? That's simply not an issue anymore in big cities. Poznan, Wrocław, Kraków and Warsaw all offer huge amounts of activities in English these days.