When an iphone shows the ellipses [...] in a messaging/text app, it does not mean the sender is typing, it only signifies the sender's [Enter Text] field is active. If they saw your messge, and intentionally or accidently taps their [enter text] bar, you will see the ellipses on your end regardless of whether or not they are actually entering text. Phones are fallible. Very likely they looked at your msg and put the phone down w/o out closing out the text msg app.