IEMs are not free from resonances but they are less prone to some resonances than most headphones.

Headphones create multiple sources of sound between the ear and the can, which can create more standing waves, out of phase cancelations and additions, etc. Some very sophisticated headphones got rid of this using acoustic meta-materials.

A good evidence of this is that the group delay for iems are usually much cleaner compared to headphones.

But resonances still exist with iems particularly at higher frequencies where the wave length is comparable to the length of the ear canal.

Nevertheless , if the measurement device is fairly accurate at higher frequencies then it would be easier replicate at least most of the sound signature, as long non-linear distortions are not significant.

Sadly, most measurements you find of the web were done with inferior couplers that make this task more difficult to do.