I have heard before that salted duck eggs are not made from fresh duck eggs, but from those 'dead eggs' that have failed to hatch, and that those eggs have failed to 'develop' once, and that the nutritional value of the egg itself has declined quite a bit, and that it may also produce some unhealthy substances. The price of such dead eggs is much cheaper than fresh eggs, the cost is low, and the salted duck eggs made into them are not expensive.

I guess the salted egg yolk you are talking about is perhaps similar to this one.