I think that what's going on is that your source has not expressed themselves very well, and you've latched on to the wrong bit of their explanation.
I think that their point was that in English since is followed by what you might call a 'location' in time, rather than (as in some languages) an elapsed period.
The 'location' doesn't have to be very specific or momentary, but it is placed in time. Examples might be:
[the time when] my father was alive
three years ago
[the time when] I moved to Yorkshire.
Star Wars (i.e. "the time when Star Wars was released, or when it became popular")
Obama (i.e. "the time when Obama was elected, or when he left office, or the whole period of his presidency")
You can see that it doesn't have to be specific at all.
I think that what your source was warning you against is things like
*since three years
*since two days
which I've starred because they're not grammatical in English, but they're something that speakers of other languages often say.