The article doesn't say the tires are not inert.

It says that the stuff they use to hold the tires in place didn't hold. So instead of a nice reef, you had thousands of lose tires smashing marine life that tried to settle there.

That looks like it failed because the bands that were supposed to keep the bundles of tires together corroded. The tires then were free to move around in the currents, which destroyed any life that had been growing on them and prevented any new life from attaching. It also let them collide with natural reefs and damage them.