This is just reinforcing a point I keep banging on and on about: talking in generalities about things like "universities" is pointless when there is such a huge gulf between the best and worst in the sector.
Quoting again from the linked substack. This quote is from the official government-commissioned MAC review he is reporting on:
"Approximately 10% of international postgraduate students in the UK who attended a university ranked between 1 and 200 (the highest ranked) globally went on to obtain a Graduate visa, whilst 30% of those who attended universities ranked 800+ went on to obtain a Graduate visa."
i.e. the people who went to the low-tier universities are the disproportionately ones using this route, not Cambridge nuclear physicists
And the huge influx of new student visas mentioned above is disproportionately going to low-tier universities. Quoting the substack author this time:
"it is striking how concentrated the growth of student visas has been among less prestigious universities. The share of visas for the top institutions had been going up. Since the graduate route launched it has plummeted"
Screenshot of that graph attached. The axes are a big misleading. The drop is from 50% to 30% but the axes make it look like it's to 0.