I would have expected this to be common when heads of state travel?

There is jamming and then there is spoofing.

I think I have seen GPS jamming in Washington DC.

What I found interesting in the report is the methods that they used to detect spoofing, namely with a receiver on the ISS.

Anti-spoofing countermeasures are an interesting topic but are classified secret so you don't hear much about it. I think one basic principle is comparing information from multiple sources. The belief is that spoofing is not so bad, even for life safety applications, so long as you know it is taking place. For instance there are multiple GPS signals and also signals from GLONASS and Galileo and a Chinese system is about to go global. Effective spoofing against a receiver that can receive them all is more complex than just spoofing one signal.

Security topics hidden behind clearances, sounds like security through obscurity to me.

It's a strange system.

Many of us self-censor because we know there are certain things you don't really want to talk about.

I was talking about a nuclear physicist, for instance, who told me privately he was concerned for years that Np237 would be the ideal material for nuclear terrorism, and I told him, yeah, I saw a paper about that from LANL, they measured the critical mass of a bare sphere, there is nothing secret about it.

As for getting a security clearance (or not getting it) many of the reasons aren't so good. For instance they may or may not have had a good reason not to give Kushner a security clearance. Many people fail to get clearance because they smoke pot or they are trans or other things that have nothing to do with "are you loyal to your country."