"All fossil fuels" is a grey area. A lot of carbonate rock doesn't make good fuel. There's also a lot of deep shale rock that might be considered fossil fuel that's unlikely to be used up because it's too hard to get to, though it can be fracked and a percentage of it could be used.
Also, the amount of fossil fuels required to be burned (and the rate of use), that might get to a runaway global warming is uncertain but could be enormous. Perhaps orders of magnitude above our current projecte rate of consumption. The Wikipedia article on Runaway Greenhouse says:
As the Sun becomes 10% brighter in about one billion years' time, the
surface temperature of Earth will reach 47 °C (117 °F), causing the
temperature of Earth to rise rapidly and its oceans to boil away until
it becomes a greenhouse planet similar to Venus today.
The average surface temperature of the Earth is currently about 16° C, and was about 15° C 50 years ago. Even the highest current IPCC projections are a far cry from the 47 degree estimate. And while there's some uncertainty in these numbers, there's at least a pretty good chance that the runaway greenhouse transition point is out of our reach.
Why is runaway global warming a possibility despite all current
fossil fuel deposits once having been in the atmosphere?
The Earth used to have a lot more CO2 in the atmosphere hundreds of millions and billions of years ago, though there is some uncertainty as to how thick the atmosphere was, so good atmospheric models are impossible, but it's likely that CO2 levels were orders of magnitude higher a couple billion years ago than they are today, but that wasn't a too much heat problem because the Sun was smaller and less luminous.
This chart gives an idea of how the transition happened, and, during Earth's past, the Earth was in fact, believed to have been a snowball for extended periods, even with higher CO2.
Also see Faint Young Sun Paradox.
There are other factors. Volcanic activity puts particulates and SO2 high in the atmosphere that reflects sunlight and cools the Earth and Earth was probably more volcanic hundreds of millions and billions of years ago. Large meteor impacts like the Chicxulub impact create flash warming followed by years of significant cooling caused by dust and ash blasted into the upper atmosphere.
There were, certainly, large enough meteors impacts during the late-heavy bombardment that the oceans boiled away and Earth was heated up and the atmosphere was hot and filled with water-vapor as well as CO2, but those were fleeting periods in geological time. The dust and debris blown into the atmosphere leads to fairly rapid cooling following those large impacts.
The sun was too cold in the Earth's past for Earth to ever experience a runaway greenhouse effect, but in a billion years or so, that is expected to change.