You are correct but the point was simply to illustrate the difference in detail that can be accomplished at various resolution which I think it illustrates… however another example such as the one you suggested would also be helpful.

Question: you have provided 2 sample picture showing the difference between a 300 and 72 DPI,
If web pages will always display pictures in 72 DPI then both pictures should have been displayed with the same quality just the 300 DPI should be strached to a much bigger size to create a 72 DPI