he is also saying that this is different to PMs - who get security regardless of risk assessments and other high profile figures
Yes, this is true. And also entirely logical.
Ex-prime ministers and probably other politicians (we aren’t told who, for obvious reasons) do get security, likely for life, after the leave office. They have actual political power in their roles, and will have made decisions and committed the UK to actions that will have made them domestic and international enemies. Plus we know very well that people target, and murder, politicians. They have a higher risk profile than other people in public life - but still, they don’t all get protected.
Ordinary citizens (like Salman Rushdie) exceptionally qualify for public security because they face an extraordinary level of threat. Generally though, members of the public don’t get protected even if there are credible threats against them, from ex-partners for example.
Members of the Royal Family also face a threat because of their role, but from the level of security that we can observe they are given, we can deduce that the view is that the sovereign, those in the direct line of succession, and those undertaking public royal engagements face the highest ever of threat. If they are not in those categories, they don’t get protection.