EDIT: Having looked into this further, I don’t think the process as described in the article is actually correct… if the root port on SW1 goes down, rather than sending a TCN, it will start sending inferior BPDUs to SW2. That’s according to this: https://learningnetwork.cisco.com/thread/63525
In the scenario given, it says that SW2 receives the TCN from SW1 and then immediately makes Fa0/19 the new root port.
A topology change doesn’t necessarily mean that the root port needs to change, so how does it know to immediately change the root port? My suspicion is that a switch should never expect to see a TCN on a root port, unless that path to the root bridge has gone down (or unless it originates from the root bridge itself). Would that be correct?