<could> for single past ability?

Hello everyone:

 According to grammar books, when you are refereeing to a general past ability, you can use both “could” and “was/were able to”, but when the reference is to a single occasion in the past, only “was/were able to”,managed to”, or “succeeded in” should be used. But while I was listening to an audible book, “English Grammar Boot Camp”by professor Ann Curzan, I heard her say “ and I could find it” in the following context.

 “Off of”tends to be colloquial, but it will sometimes  appear in more formal writing. I went and searched in the Corpus of contemporary American English and I could find it in academic prose…….

As far as I know you should say: I ran after the bus and was able to catch it, not [ and I could catch it].

 Shouldn’t Professor Ann Curzan have said “…and I was able to find it?

 Thanks

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