Why is runaway global warming a possibility despite all current fossil fuel deposits once having been in the atmosphere? Disclaimer: this question is not about climate change or global warming in general, and does not try to dismiss general human-caused global warming.

The concept of runaway global warming depicts a scenario where the climate enters an unstoppable positive feedback loop, and this scenario is often depicted to happen before all fossil fuels have been used up. This topic often comes up in discussions about how long fossil fuels will last, with a warning that the planet would be uninhabitable if (or before) all fossil fuels were used up.

However, all these fossil fuels were once in the atmosphere in the form of CO2, before they ended up absorbed by plants. Why didn't Venus-like conditions form back then, and since they didn't, why could the result be different in the future?
The largest reservoir of carbon is the rocks (sedimentary rocks) which reach into the order of the million gigatons (Gt). Over millions of years, part of that carbon have been released to the atmosphere by volcanoes. However, the amount of carbon in the atmosphere remains fairly stable because chemical erosion of rocks and plants capture that carbon excess and put it back in the geological reservoir.
There is about 4.700 Gt of fossil fuels, and the Atmosphere contain only 760 Gt of carbon.
So, if we burn all fossil fuels slowly, lets say over many millenia, there is not a problem, plants and erosion will keep the balance.
However, if we burn it quickly, let's say over few centuries, the greenhouse effect can be so strong that can kill many plant ecosystems and rise sea level, therefore reducing the surface area of rocks exposed to chemical weathering. Both effects would reduce the capacity of the system to restore the equilibrium, therefore it could take many millenia before all that carbon is returned to the geological reservoir so that temperatures can return to normal.
An extreme runaway case, like what happened in Venus, is unlikely to happen on Earth, but we don't really know. If temperatures ever reach to the point that oceans boil, then, there is a possibility that Earth might become like Venus forever.