They are two different things -
Thallium, we see whether the blood reaches the muscle / it will give clue - the muscle is living or dead.
is the gold standard - that only will show = whether there is obstruction / if present, where and how severe they are / the function of chamber and so on
The difference is like seeing a shadow and the substance - thallium is like seeing the photo / angio is seeing the person
Is it now clear? The aim of any investigation is to modify the treatment based on the results. You must be prepared for angioplasty
or bypass - if the angiogram shows obstruction. And this seems likely in your father's case - even though, you have not stated the report of thallium...