You can see the full work in his other blog post[0]. There you can clearly see that red circles coincide with vertical blue lines on the graph. And the very fine print in the bottom left corner explains that "Displayed years indicate when Coffee Maker computers were built". Overlapping the red points with title text is probably just a stylistic choice.

[0] https://www.dougmacdowell.com/hand-drawn-data-visualizations...