I may be wrong but Fit Text to Curve in XWD465(P) is not possible when it is a Smart Shape Line as it destroys the Smart Shape and returned back a Text line.

In XWDX, you have Open SmartShape, where you can add the text and still preserve Smart Shape functionality.

Unfortunately the text does not sense how long the line is so you have to adjust with the Text Tool.

Xara, as a minimum, should offer text on Smart Shape lines and if Xara can add controls to fix text left arrow, right arrow head, both and centrally then we are cooking.

Text on a Smart Line.web

Acorn