The idea is that, if you're an American, you could get PR citizenship status with one year and then Spain would give you citizenship with only two years. You might consider the PR citizenship to not actually be a "citizenship" but merely a residency status (like how you might live in California or Washington).
Though technically, US citizens are also citizens of the state in which they reside. So, it's not really that different than what states do when considering things like in-state tuition at schools. So, yes PR has citizenship just as California and every other state has citizenship. I guess the difference is that PR considers that you don't lose the status of PR citizen when you cease to be a resident (at least if you were born there or a parent was).
Spain is also free to make the laws it wants to access their nationality. They could decide that those born in Florida should have easier access given Florida's historical connection to Spain. France was at least considering allowing anyone born in a place that used to be French territory to get citizenship. I believe Bill Clinton joked that he could move to France and run for President there since Arkansas was part of the Louisiana Purchase from France.
Anyway, it seems that PR citizenship needs US citizenship, but US citizens can gain PR citizenship with one year of residency.