Why was the cooking liquor for my mussels too dilute?

Yes, the additional liquid came from the mussels and is going to enhance the flavor of your finished dish. All you need to do is remove the mussels to a bowl, cover lightly to keep them warm, and then turn the heat up to high on your pan and reduce the liquid until it's concentrated before whisking in your butter. (I presume you added butter even though you didn't list it above?)

If not...finish it by whisking in softened (not too soft, but slightly) butter (known as "mounting with butter" monte au beurre) which will emulsify with the concentrated cooking liquid and add a luxurious texture and richness to your sauce. Finish by adding your chopped parsley and adjust seasoning according to taste.

Mussels absolutely release liquid when cooked. You might try a thicker sauce? Reduce the wine a bit? Seems like 200ml should be about right though...Most recipes I've seen call for 200-400ml.

Like @Fernando mentioned I always add the mussels to a hot saute pan with olive oil, S&P and garlic and saute them for around 20 seconds then add the wine letting it reduce a little and at the very end pull it off the heat and throw in a little butter along with the herbs.

The butter will form an emulsification with the wine reduction and give you a wonderful sauce.

You dont say how many mussels you are cooking, it could be you are adding too much wine for the amount of mussels.