Yes I’d say you’re misreading it (or rather they’re just doing a bad job of explaining it). What they’re saying is that if you get 20% of atmosphere at sea level, when you’re in denver you’ll get 20% of the reduced Denver atmosphere.
20% of 29.92 = 5.984 absolute
20% of 24 = 4.8 absolute
So you can see the vacuum is actually deeper in Denver. However what the gauge will read is:
Sea level: 0 - (.8 * 29.92) = - 23.936 gauge
Denver: 0 - (.8 * 24) = -19.2 gauge
So the vacuum will look worse on a guage in Denver, but actually, it’s better.
The guide is simply pointing out that you need to do this math when selecting a pump to determine appropriate sizing. Notice in the last sentence they actually switch to a totally different problem, and ask you to hit an arbitrary vacuum capacity in Denver. The question was not whether the new pump was bigger than in problem 1 or not.