I've just realised that this lesson needs more detail to cover ser and estar.
When talking about something earlier in the day, generally, something that uses estar will use imperfecto, not perfecto nor indefinido, breaking the rule completely! Whereas ser will follow the rule.
So if we ate something delicious this morning we'd say, "!Estaba rico!" not "he sido rico" nor "estuvo rico".
Estar by its nature expresses more transient states of being than ser and the imperfect tense matches this (at least, that's how it feels to me but as I'm not a native Spaniard, I look forward to hearing Kwiziq's native experts' views to expand or correct this for me!)