It sounds strange to me to say Ellos fueron amigos. Isn't it better to say Ellos eran amigos, when there is no time specification?
I imagine you are referring to this sentence in one of the questions for this lesson:
It is, as you say, more common to see and say "él era mi amigo" using the imperfect, but we can also use the Preterite tense here. In this case we are seeing this "friendship" as something that took place and finished, had an end; therefore the preterite makes sense.
If we say "Él era mi amigo", then we are not seeing this friendship with an end. It is simply describing a situation in the past, more as an ongoing action.
We have a lesson on using SER in the preterite and the imperfect and their difference here.
I hope this helps and clarifies your doubt.
Un saludo cordial
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