it's close. Depends on how important it is that you lessen the risk for flat on your next ride. For my everyday rides, I can risk a flat. So I'll ride a tire till the threads are showing. Though maybe the threads are showing there, it's a crack that has rubber over them. And not the rubber worn down to the threads.

Now if you install the tire and when pumped up to pressure the tube is exposed and coming out in a bubble, then that's a problem and indicates a new tire. Unless you want to boot it with something. I'd toss it at that point.

If your next ride is important, or you are riding with a group that you don't want to risk inconveniencing and being poked fun of, then change the thing now.

It's mostly just what makes you feel good and the depth of your pocket book. It's only a flat you'll be risking. I've never had a flat cause a accident. And I've had some impressive blowouts over the years. So I can't say it's unsafe.

Of course if you are in Portland OR, and have gone up a mountain, then consider your confidence in the tire when coming down the mountain and having several switchbacks with sheer cliffs off to one side.