Plants aren't the only things that sequester CO2, animals in the ocean do too and that gets deposited on the sea floor when they die. It's believed that during the Cretaceous, the CO2 levels were over 1000 ppm which contributed to a warm climate and huge plant growth. CO2 is around 400 ppm right now, so something sucked it back out of the atmosphere between then and now. And obviously the extremely high CO2 level didn't lead to runaway global warming then, so it's hard to see why it should do so now at a claimed "point of no return" that's much lower than 1000 ppm.

But really my position is we just don't know what's going on. IMO it's the only honest scientific position about the earth's climate. We're talking about a very complex system with very long term cycles we don't understand and can't measure short of spending thousands of years on the problem. Of course we don't understand it; we're just guessing about most of it.