How many people are on their team? Do we know that? I assume they meant their editorial team. For a new imprint, even at a large publisher, that's likely to be only four or five people at most -- who all, as chopper suggests, have to spend large parts of the day doing other things, because they can't just forget about all the other books they've bought since they started the imprint, books they have to shepherd through the editing and production process.
And all those other things they do depend in part on other people getting their jobs done on time and getting things right. And when they don't, then there have to be conversations, and things sent back until they come back right. (Maybe sent back more than once.) Before a final decision is made to buy a book several people have to read it: maybe the whole editorial team, likely people from sales and marketing as well. So it's not just x number of editors dividing up the work. With the ones that are rejected immediately, yes, one person reads the manuscript and makes the decision. But for the books that are seriously under consideration the job is not divided up between x editors; each of those submissions will have to be read x times. Maybe one reason they are taking so long is because they have received so many submissions that are good enough to pass around to the whole team, more of these promising manuscripts than they anticipated. Editors get sick and can't work for a few days; then when they come back they have to play catch-up with all their other obligations before they can even look at the open-window-submissions again. Authors call up to talk about their insecurities and need to be reassured. Agents call up to talk about their authors.
None of this can be predicted in advance, or how much time it will involve. Which is why they may promise to get everything read by such-and-such a date and respond to all the authors, and really believe that they will get it done when they say they will, and then all these other things come up and they can't.