"Je fumais depuis des années quand j'ai décidé d'arrêter"

Question

I don't understand why "depuis" is used here despite the fact that we use "depuis" for ongoing actions as stated in the lesson below.

Using the present tense (Le Présent) - and not the compound past (Le Passé Composé) - in sentences with "depuis" (since/for) in French (French Prepositions of Time)

Accepted Answer

This is an “advanced” example - not “has been happening since” but “had been happening since”.
In this case the ongoing action was in the past. It was interrupted/stopped by another event - described in passé composé. 
2 useful links below - look at “has been happening” and “had been happening” explanations especially.
 
https://www.thoughtco.com/depuis-pendant-pour-1368831
https://www.lawlessfrench.com/grammar/depuis-vs-il-y-a/

Answer

Je fumais depuis des années quand j’ai décidé d’arrêter. — I had been smoking for years when I decided to stop. Note that in French you use imparfait where in English you find past perfect tense.