I was taught that both "was/were able to" and "could" can be used to express ability in the past.

Roughly speaking, "could" is not used with reference to one (or more) specific achievement in the past, but it can be used when referring to a general ability in the past (this, I think, is the case with your example from the Longman dictionary).

This is what Swann (in his "Practical English Usage") says:

M. Swann

Ҥ 122.4 past: She could read when she was four

We use could to talk about the past.

    She could read when she was four.

    My grandmother could sing like an angel.

    My last car could do 200kph.

    In those days everybody could find a job.

Was able to is also possible, especially to talk about people's ability.

    She was able to read when she was four.

§ 122.5 past: Could is not always possible

We use could for general ability -- for example to say that somebody could do something at any time, whenever he/she wanted.

    When I was younger, I could run 10km in under 40 minutes.

We do not normally use could to say that somebody did something on one occasion. Instead, we use other expressions.

    I managed to run 10km yesterday in under an hour. (NOT I could run 10km yesterday ...)

    How many eggs were you able to get? (NOT ... could you get?)

    After six hours' climbing, we succeeded in getting to the top of the mountain. (NOT ... we could get to the top ...)

    I found a really nice dress in the sale (NOT I could find ...)

However, we use couldn't to say that something did not happen on one occasion.

    I managed to find the street, but I couldn't find her house.”

You might want to read these two posts written by CalifJim:
Ability of modal 'could' to denote abillity
Modals 'could' and 'can' to denote possibility