Chickens are good at managing their own calcium levels when given the option. With layer feed the calcium is mixed in so they are not able to scale back that amount if they are getting too much. If your hens are all laying well and regularly then the amount of calcium is fine but with mixed flocks, molting birds, old birds, young birds, birds that lay infrequently, roosters (and on and on lol) that amount of calcium in a layer may be too much.
If they are not needing calcium they won't eat the oyster if it separate. For example, I recently refilled my oyster container but my girls are molting right now and it remains untouched (with a grower feed).
There are two schools of thought on feeding layers and people have really strong opinions on it. You can either go with the layer and then you really don't need extra calcium. Or you can go with a flock raiser, grower, or game bird feed and do the calcium on the side. There are two main factors to the choice: calcium and protein. Layer feed has calcium mixed in. Other than that is it normal feed with usually around 17% protein. Non layer feeds are available in different protein percentages from 16-24ish. Also any feed can have soy protein or animal protein. I personally have found it harder to find a layer without soy but I live in the city and my local opinions are limited.
It;s not as complicated as it sounds though. I would read the labels on your feed carefully so you know what you are feeding and decide what you think is best personally.
I go with a grower + oyster. I have a silkie who rarely lays and in times like this when my girls are molting and are clearly indicating they do not need extra calcium, I am glad it is not in their food. I have one bird that lays thin eggs no matter what she eats (and I have tried lots of variations) but on the whole I have never had thin shell problems and get beautiful thick eggs on the grower feed.