Not even close. Second law of thermodynamics puts a limit on heat engine. Gravity storage is not subject to such limit.
But it is a great idea to burn any fuel whose oxidized product is not a gas. The only reason why burning hydrocarbons is problematic is because carbon dioxide is a gas, otherwise it could be captured and recycled like this.
Right, conversion of iron to rust yielding heat is 100% efficient, less only the work needed to move the rust out of the way.
If you wanted to convert that heat back to electricity, there would be losses, but nowhere near as much as some people are saying. 1800 degrees minus ambient is a very big delta T. You are not bound to a closed cycle, so Carnot does not apply. So it is likely 80+% would be achievable, maybe over 65% round trip.
Gravity storage IS subject to the same law, why do you believe it isn’t?
Burning hydrocarbons is problematic for a host of reasons besides CO2 release: fracking, oil spills, release of other pollutants, political issues with the supplier countries, pipelines, ...
Gravity storage converts work to potential energy, and back, efficiencies can be 90-almost 100%... heat engines at room temperature generally top out in the 35%ish range. Using the "waste" heat improves that, but nowhere close to 90%.
Because the Carnot limit is particularly pessimistic.
70% vs 40% is a big difference, especially when you consider it from the losses column instead of the wins column. If I gave you a system that was losing 30% of a resource to replace a system that lost 60%, you've cut your losses in half.
If I told you a mechanical system could get that to 15%, you'd replace that again. Anyone who tells you they can get a heat engine down to 15% losses is a charlatan and should be reported for fraud.
But it is a great idea to burn any fuel whose oxidized product is not a gas. The only reason why burning hydrocarbons is problematic is because carbon dioxide is a gas, otherwise it could be captured and recycled like this.