"Dielectric nipples" are a marketing term for the most part. They are NOT truly dielectric, because you DO have the dissimilar metals in contact with each other. What these nipples to is prevent the worst EFFECT of having dissimilar pipes in contact, that of the pipe closing off from the INSIDE with corrosion buildup, which you can't see so you don't know it's happening until the water stops flowing. That's because it has a PEX (high temp. plastic) liner on the inside. But because the dissimilar metals are in contact, the corrosion happens on the OUTSIDE, as it is in your case. However, now you can SEE that and replace them when it happens.
The only thing that stops the electrical flow is a dielectric UNION that puts a non-conductive material between the metals. Even then, if the water has a lot of dissolved minerals in it, internal corrosion and blockage can still happen because the electrical flow takes place through the water. So the best solution is BOTH the union and the sleeved nipples.
Or like I said, when you see the external corrosion, consider it a warning and just replace them. From the photos you posted however, I don't see any kind of union fitting anywhere, so replacing them is going to be difficult. If there isn't one ahead of those pipes somewhere out of the photo, you are going to have to cut or at least de-solder the pipes. Not a great idea for the uninitiated. But if you do, put in a union so you don't have to do it next time, and if you do THAT, make it a dielectric union.