Why don't you give the appropriate question to the answer she made? She is asking if there is a function in Affinity Publisher to convert a shape into another, like InDesign does (Search InDesign > Convert to Shape tool). What you suggested is to use the border radius effect to change make the square look like a circle, but:
1. it is not a circle, is still a square, so it will act a s a square even if its appearance is similar to a circle.
2. This is a solution to solve this specific task. But what if I have to convert a circle to a square, or any other shape into any other shape, for example a triangle to an hexagon, and so on...
So the answer is NO, unfortunately, there is NO convert to shape function. And this is a big lack in a software for the professional desktop publishing which aims to replace or at least give an affordable alternative to Adobe.
As experienced designer (20 years adobe user, ACE), I wanted to give it a chance testing the trial version of the Publisher and the Photo. I found them really good softwares with a lot of fast and intuitive and time saving tools (and some unexpected really cool stuff that I always dreamt for Adobe), but I think they still miss something if they want to become the real alternative to Adobe for all the designers of every kind out there.
Anyway what I wanted to say is that I get really nervous when searching in forums I find those kind of answers.
THE QUESTION IS SUPER CLEAR SO PLEASE A SUPER CLEAR ANSWER: no there is no such function in Affinity Publisher....punto!
Grazie.