I will say it depends on how you define sharp and how clean you keep your gear.
If you like to keep your knives really REALLY sharp and have sharpness tests you like to perform (beyond answering the question, "will this cut what I need to in the field?"), then yes, the sheaths will dull the knives a bit. We are talking about silly levels of sharpness here.
Additionally, if you get debris down in the sheath and fail to clean it out, it can cause some dulling issues.
HOWEVER, I have never found a Kydex or molded plastic sheath that would not do this to some degree, and ESEE sheaths are among the best I have experienced, and they are among the easiest to clean and keep clean...so if you are not a total dolt, you ought to be fine.
If you use your knives in the woods and don't worry too much about sharpness tests, you will likely conclude that your knives will stay VERY sharp while using these sheaths.