The note in the lesson is saying that "Ser (to be) is not followed by preposition "a" but Ir (to go) is followed by preposition "a".
In the sentence "Yo fui a la fiesta", fui means "I went..." not "I was".
I hope this clarifies your doubt.
I do apologise. You are absolutely right. That question is showing a different preposition after "fui" as "went" and that is very confussing. I will remove that question from that lesson.
For your information "ir de fiesta" is an idiom which means "to party", that's why.
I hope that makes more sense now.
Thank you very much for reporting this.
"Fui de compras" I wnet shopping
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