Their education mostly or totally happened at the expense of other states and the way they're being treated means that they don't get adequate recognition for their work, so yes, exploited.

They can go to work in other, more civilized, western democracies where they're not made to feel unwelcome. Standards of living in the UK aren't that special and these people are there because they're tied to specific research projects, not because it's a great country.

Perhaps GP is referring to Marx's theory of exploitation in which surplus value is appropriated from the workers. Many people, especially in jobs which require a lot of equipment (e.g physics, theoretical physics) can't enter self-employment or become bourgeoisie themselves, in which case they are almost forced by societal structure to stay as a member of the proletariat and thusly paid wages for labour-time during which they perform extra labour which is collected as surplus.

That's a reasonable take on what GP was referring to, though I'm not sure if that's it. Exploitation of human capital is everywhere, whether in Britain or China. The standards of living and 'civilised Western democracy' are meaningless and irrelevant to the theory of exploitation.

The political climate in the UK is currently leaning towards being hostile towards benefits being paid to EU citizens.

Why should I pay taxes for benefits that I can't use?

I'll go somewhere where I'm wanted.

It seems they are reconsidering, so no, not voluntary. They voluntarily started worked, now things have changed and they do not think that was a good decision.