So as mentioned by UiPath for input dialog, If set to contain only one element, a text box appears to write text. If set to contain 2 or 3 elements, they appear as radio buttons to select from. If set to contain more than 3 items, they appear as a combo box to select from.
Unfortunately, this behaviour is hardcoded into the activity, so you cannot force radio buttons for more than 3 options using the default Input Dialog.
So you are right in your understanding!
Only Work Around is Forms