If you can see that lock then you can pick it easy enough... hanger, flip, done. So not even remotely secure at all against an adult who might try to come in.
Which is why MY first thought was "Do they have kids"... of their own or visiting (any relatives come for the holiday?) or babysitting ones... that might see that door and decide to explore... short people to who that little hook is the devil because THEY are too short to reach it and thus it keeps them out??
I've used those myself, to keep my kiddos in/out, work brilliantly... until they get big enough to pull over a chair anyways... and they're uber cheap and easy to install.
Or for dogs come to think of it... Janie will nibble JUST right on the bottom corner of the door and can pull it open. Can't do that if it's shut totally, but with a closed but not totally closed (laziness or warping) she can get anywhere... so that's a thought too.
If any of those are possible I'd prolly convince myself that was it... based on my own experience, and the total lack of security with such a flimsy thing... not like it's really going to keep anyone out... I reckon it's at least sorta logical.