If a new Democratic district is created, those Democrats must be taken from somewhere else. It is quite possible that in the process of creating one new Democratic district, two or more districts will be tipped toward Republicans.Silver is explaining how complicated it is, but he's actually also oversimplifying, because he's assuming each vote is either a Democrat or a Republican. But if you set up a district with 45% Republicans and 55% Democrats, the Republicans might be able to win with a relatively liberal candidate, especially if the Democrats had a candidate who leaned too far to the left. How safe do you want the district to be? If it's super-safe, you waste votes, but the narrower you make the margin, the more likely it is that the other party can swing enough of your party's voters to win.
The key is how efficiently each party’s voters are allocated. What a party would prefer is that, in the districts where it has a majority, that majority is as small as possible, so as not to waste any of its voters.... Conversely, in those districts where it didn’t have the majority, it would prefer to lose by as many votes as possible — in fact, it would prefer to have none of its voters there at all.
What a party wants to avoid, meanwhile, is districts where it has say 45 percent of the vote: it’s using up a fair number of its voters, but not enough to give it a majority. It would also like to avoid districts where it has close to 100 percent of the vote, since so many of those votes will be superfluous.
How predictably Democratic are Hispanic voters? As Silver notes, they are not as locked in for the Democrats as are black voters. Silver says that's what makes Hispanic-majority districts more helpful to the Democrats than black majority districts: A majority minority district can be created without "wasting" as many Democratic votes. That only works, of course, if these Hispanic voters still go for the Democratic candidate.
Silver doesn't talk about the fact that the GOP controls the Texas legislature, so it will dominate the decisionmaking about where the district lines are drawn. It may be able to craft majority minority districts that have a close enough political balance to allow them to win, or it may be able to figure out how to pack the consistently Democratic voters into one district. It's a subtle game, and the parties have gotten really good at playing it over the years.
ADDED: Please note that I've expressed no opinion about whether the Voting Rights Act actually does require Texas to make 3 of the 4 new districts majority Hispanic. I assume there will be plenty of litigation over this.