According to the article it took 7-8 rounds to stop it. If I was going to use an AR15 for bear I would want to upgrade to a larger caliber. That said, griz have been killed with 22LR also. You use what you have when you have to.
I’m still hanging on to my M-1 Garand with a bandolier of ammo right at hand. Killing shot right through a plastered wall? Probably.
The AR-15 is one of the most modular and versatile rifles ever designed. I have 5 AR-15’s. One is chambered for .22 LR, another in .300 Blackout, another in .458 SOCOM, and the other two for 5.56 NATO.
The .458 SOCOM is more than adequate for potential one shot stoppages of a Grizzly bear inside of 100 yards. The great thing about modifying the AR-15 for all of the above mentioned calibers is a barrel swap for the .300 Blackout, (no other parts need changing, still 30 round capacity, ballistics similar to the 7.62x39 AK-M round) a barrel, bolt and bolt carrier swap for the .458 SOCOM, (300 gr. 2,045 ft-lbs energy, similar to the Govt 45-70) and an upper receiver and mag change for the .22 LR. That is it. You can then hunt every game from jackrabbit thru Grizzly Bear with the same basic rifle platform.