This is not true. Here, I took these screenshots for you where I copied an unsigned binary to a random location and forced a UAC prompt:

https://i.imgur.com/BSJlSAf.png

https://i.imgur.com/4ZVNsPN.png

I also did the same with a signed binary:

https://i.imgur.com/xpSiMMY.png

https://i.imgur.com/ACLydv0.png