Yeah if the vet did an ultrasound and found fluid for example it might be hard to say whether it is fluid from internal laying or from ascites from the liver without doing blood tests to check the liver and heart function. If there is soft tissue it might be hard to say whether it’s cancer or inflammatory material from the oviduct etc. It can be difficult to know for sure and tests rack up the costs very quickly unfortunately.
From what I’ve read about EYP it’s fluid build up from the yolks but over time the yolks harden and then any infection is going to cause an inflammatory response.
If your hen is laying, eating and drinking, her poops are normal and her crop empties overnight, that’s the best you can hope for. The implant can be a good preventative and/or treatment for any number of reproductive issues so if you’re worried about her but don’t have the funds to get everything tested to be as certain as possible, it’s a good option to take.
I lost a hen recently who always had this bulge under her vent and I was always concerned it was something but never knew what it was — hernia, fat, cancer — and upon necropsy there was nothing out of the ordinary in her abdomen or organs so it just goes to show how individual , random and mysterious these creatures can be.