It's not even assumption which is guaranteed to have a unique answer, much less the correct unique answer. It's possible for a person to experience the same time local time (on the same local date) in more than one time zone when travelling.
Heck, it's possible to experience the same local time on the same local date twice without travelling during DST or Summer Time transitions, and while most people don't have events scheduled then, people who do might work might.
Suppose I'm coming back to Chicago from a trip to Japan, and a friend wants to get an early dinner when I get back. One frequent direct flight from Tokyo leaves NRT around 6 PM and arrives at ORD at 4 PM on the same calendar date. Before I leave Chicago I set an event for meeting them at our favorite restaurant at 5:30 PM. Should my calendar remind me about the event when it's 5:00 PM where I'm at waiting for my flight to board at Narita or at 5:00 PM where I'm at back in Chicago?
The author says you should assume the time zone of where you will be when that time occurs, but that's certainly not a safe assumption, as I may schedule a call at my Boston office for a certain time, but then may be traveling during that actual trip but the time should still be Boston and not where I'm traveling. I use Google Calendar backend but only interact through my Mac or iPhone using iCal or Fantastical both which support time zones. So for most people who time zones isn't an issue, its not a problem. For people who travel often or who have clients in different time zones, you simply add a couple keys when creating a smart entry in Fantastical. Just like when I'm talking to client who know are in a different time zone I always mention the time zone after the time.