Something to keep in mind is that companies really want to hire good people for the positions they are looking for fill—and it’s really hard to do that based off only what’s on paper and what can be gleaned in a few brief interviews. Obviously, that’s usually all they have and they’ve tried to get good at using those limited opportunities to make good decisions, but that kind of process—even some of the more absurd processes thrown together by some firms trying to be “selective”—are never going to give you the same kind of information you get from working with someone day in and day out. And ultimately, that’s the information they want, because they want this person to work day in and day out and be productive and get along with the team.
That’s where a personal recommendation can carry a lot of weight: it can confirm that what looks good on paper, what sounds good in an interview, actually works out well in practice. It means something if you say “I worked next to this person for X hours a week for Y years, and would happily work alongside them X′ hours per week for the next Y′ years.” It suggests that there aren’t any hidden pitfalls with the candidate, they aren’t going to be deadweight or impossible to work with.
Would it mean more if you had been there longer? Of course. But it certainly doesn’t mean nothing as things are. Really, it’s your experience with him—rather than your experience with the company—that matters most here. They already know the company, and thought you were a decent fit for the job. What they don’t know is him, and you do. And by putting your name (and thus, reputation) on it, that likely gives them a reasonable amount of confidence, so they don’t fully need to “trust” you—the potential harm to your own career should be enough to keep you “honest” and they know it. (Quotes used because it’s not really a question of trust so much as confidence, but I’m not thinking of a better way to articulate things.)