Hi KayJazz,
This is “partly” correct but is also “partly” incorrect.
The ILD of a material is not the only specification that has an effect on pressure relief and the sensation of softness (see post #4 here).
If you sink into a mattress layer exactly 25% they would be the same but in most cases different parts of the body or different people will sink into a layer either more or less than this. A material that has a higher compression modulus (gets firmer faster with deeper compression) will feel firmer if you sink into it deeper than 25% and softer if you sink in less than 25% compared to a different material that has the same ILD.
It’s also not the case that all ILD measurements are done in the same way because they can use different perentages of compression to measure ILD and they may also use different layer thicknesses as well which will all affect the ILD rating. There is more about the variations in ILD and how it is tested in post #6 here.
There is also more about the difference in “feel” between Dunlop and Talalay in post #7 here.
Phoenix