The article mentions corrections beyond 500ppm. I worked on time synchronization (using PTP instead of NTP), and when we are talking about "cheap oscillators" we mean those specified at 50ppm (50us per second) which you find in every Ethernet card. In reality we measured +/- 8ppm. Yes, the drift changes over time, and you can see in the log when a door is opened in the morning. But NTP can only reach milliseconds precision anyway, so there is plenty of time to react. Whatever the problem is with those PCs mentioned in the article, using a temperature-controlled oscillator would be overkill.