As you noticed from AF1, the people who manage the flights of nation-level VIPs are extremely cautious and have a rich, well-informed risk model. You can imagine these airplanes have many countermeasures (SPAR19 is a C-40C, which is a military version of a 737). For all we know it was a decoy plane.

Tracking the plane is probably very trivial for the Chinese army so hiding the plane from public wouldn't help

Flight radar isnt exactly live. There is a slight delay if you compare flights over your head to what flight radar is telling you about those flights.

Cannot judge if this is a joke or serious theory?

What's the risk of a plane flying over the Sea above the reach of "simple" weapons is being tracked?

A military-level attack against the third highest representative of the United States is a quite certain way to launch a war, maybe even nuclear retaliation, based on how quickly and well "prove" for attribution is unveiled.

However getting attention is a key purpose of that trip.

When US representative fly to Taiwan secretly it's disabled but whole world was talking about Pelosi going there and China's treats to shoot her down for day or two. Putting that flight for whole world to see is actually more secure than trying to keep it secret.

It would be a lot more dangerous for Pelosi if it weren't.