Imho, an excuse is just an invalid reason, and that varies person to person and situation to situation.

I work in a role that has customer service elements, and sometimes there is no difference. Some people just want to be angry. CEO types are the worst about this, where an “excuse” is any less than perfect reason (from their ungrounded point of view) and a reason is whatever they accept, and usually their mind has already been up. Perfection or perish.

On the other hand, if I was late to work, and my manager asked why, and I said “Oh, there was a major wreck on the freeway” he would say “oh, that’s fine” knowing I often get to the office a little early because I add in an extra 20 minutes for traffic, since that usually gets me into the office in time due to how unpredictable my commute times tend to be.

The director at my first job, would not give a flying fuck, and would exclusively say “stop making excuses, you should have left earlier” if i was like two minutes late for any reason, even if a normal person would go “oh ok, that’s fine”