Thanks for posting the coil spring links to alibaba.
I do not think that showing these coil spring antenna relates in any way to the coil spring in mattresses acting as antennae.
I admit to not being an expert in such matters, but I would think that the plethora of springs in close proximity, the difference in sizes, the complexity of the metalwork inside a mattress would annul any possible EMF re-transmission. Having tested it, I know that it doesn’t change my mobile phone signal.
The only way to prove that the mattresses actually do transmit EMF is via proper testing in a controlled environment. I had a quick search and couldn’t find any valid studies on this topic from a reputable & independent institution. Have you been able to find any?